To answer this question properly for the present times, we have to first understand that are three kinds of Gurus in this world: Uttama Adhikari Guru, Madhyama Adhikari Guru, and the Kanishtha Adhikari Guru.
Gurus are never differentiated or discriminated on the basis of male or female or any other bodily designations for that matter.
Lord Gauranga Mahaprabhu Himself confirmed this in Caitanya-caritāmṛta 2 8.128):
kibā vipra, kibā nyāsī
śūdra kene naya
yei kṛṣṇa-tattva-vettā
sei 'guru' haya
“One may be a brahmana, sannyasi, outcaste, or anyone for that matter. That does not matter. If one has realized Krishna-Tattva, he or she can become Guru." (And to the extent one has realized this Krishna-Tattva, one is an Uttama, Madhyama, or Kanishtha Guru.)
I am fully aware that in the traditional Vedic system, matajis are always protected by their parents in childhood, by their husbands in youth, and by their children in old age. And the husband is supposed to be their second guru. But it is not always so black and white in Kaliyuga as the husband may not be always a more advanced devotee than the wife or may not be even a devotee at all.
More than that, the compassionate principle of actually sharing Bhakti Yoga is higher than the highest Varnashrama principles, because it is more on the eternal platform of permanently saving the souls from the cycle of birth and death.
So if more new souls can be reached and helped to take up Bhakti Yoga by matajis becoming Diksha Gurus while following the regulative principles, I am all for it. Because this topmost principle of JIVA-DOYA exceeds all the other principles of even Daivi Varnashrama, Grihastha, Sannyasi, and Brahmachari Ashrams.
Unfortunately in my experience, Shiksha Gurus are not all that respected even though they are the cause for introduction of a soul to Bhakti Yoga, especially after that formal Diksha fire-sacrifice. Srila Prabhupada writes in Chaitanya Charitamrita that a constant Shiksha Guru should slowly go on to become the Diksha Guru of a disciple, as that personal faith and relationship of a teacher and student is already there.
Due to the traumatic influences of Kali Yuga, personal guidance is slowly becoming a very important part of the Guru disciple relationship. Even though transcendental Vani sanga should be enough to nourish the disciples, unfortunately the present-day disciples don’t find it enough and need a lot of personal spoon-feeding. In such a situation, female Diksha Gurus may fill in this void especially in regards to female disciples.
Why Kanistha Gurus Are Required?
In the present rapidly degrading Kaliyuga, many of the Gurus are Kanishtha Adhikaris. Even if there is a risk of them falling down later, it is still better than having no Gurus at all in this world or only a handful of Uttama Adhikari Gurus or Madhyama Adhikari Gurus in the whole world who may not be found by or be approachable to everyone.
Five hundreds years ago, when so many Uttama Adhikari Gurus or Associates of Lords Nityananda Gauranga were physically present, then it would not have been right for Kanishtha Adhikaris to assume the role of serving the souls by becoming Diksha Gurus. But the situation is very different now as truly Uttama and Madhyama Gurus are very difficult to come by.
At least these many Kanishtha Adhikari Gurus can raise the disciples (millions of souls) from a prior fallen state of -50% (just for example) to 20% (the current spiritual level of Kanishtha Adhikari Guru for example), even with their not fully perfect spiritual guidance.
From there, they can be further raised to even 100% by hearing or reading the pure Vani of Uttama Gurus or Acharyas. Yes, that is the power of pure Shiksha, as the Shiksha Guru is the personification of Sri Sri Radha Govindadeva, who engages us in Their service (seva/abhidheya).
When there are millions of prospective disciples, at least tens of thousands of Gurus are required for initial upliftment even if they are Kanishtha. Because it is very difficult for one Guru to have more than a few thousands of disciples, especially if one is a Kanishtha Guru.
Why One Cannot Just Accept a Non-Physically-Present Uttama Diksha Guru?
One may say why not just mentally accept an Uttama Adhikari Guru from one of our Acharyas in the past. Isn’t it better than accepting a living Kanishtha Adhikari Guru in the present who may be flawed in some way or may fall down in the future? This kind of thinking is not in accordance to the scriptures because Diksha has always been accepted from a physically living Guru in all our scriptures and by all our Acharyas.
Srila Narottama dasa Thakura accepted a physically living Guru even though all the love was transferred to him directly when Lord Gauranga Himself embraced him. So did Srila Srinivasa Acharya even though Namacharya Srila Haridasa Thakura and the Srila Rupa and Sanatana Goswamis appeared to him and embraced him directly.
One may then argue that these were all living Uttama Diksha Gurus which were accepted not Kanishtha ones. So this does not apply now when Uttama Diksha Gurus are very very hard to find and this justifies accepting a Diksha Guru posthumously.
This claim is refuted in Srila Narahari Sarakar's Krishna Bhajamrita which clearly states that one may have a Kanishtha Adhikari Diksha Guru. In fact is stated that if one gets higher Shiksha from an Uttama Adhikari Guru, then one should actually come and share it with such a Kanishtha Diksha Guru, but never leave such a Kanishtha Diksha Guru, unless the Guru becomes envious of other pure devotees. To illustrate this point, an example is given in Krishna Bhajamrita of how a son goes to a rich person to get wealth and then comes and shares it with his own poor father but never leaves his father because he is poor.
Why Accepting a Kanishtha Guru Does Not Mean That You Will Remain a Kanishtha
One should clearly understand that accepting a Kanishtha Diksha Guru does not necessarily limit the advancement of their disciples only to the level of their Guru. Because they can always directly associate with Uttama Acharyas and Lord Nityananda Himself through their Siddha Vani and sound vibrations. Thus it is quite possible that a disciple of a Kanishtha Guru can become more advanced than the Guru. In that case, this disciple should come and share this advanced mercy and realization with one’s Guru as described above in Krishna Bhajanamrita.
Even if the Diksha Mantras received from a Kanishtha Guru may not be fully imbued with the Forms and Pastimes of the Lord, one can always read or hear the perfect Writings or Kirtan recordings of the more realized and siddha Hare Krishna Maha Mantra, Pastimes, Sound Vibrations, etc. from a more advanced Shiksha Guru or Acharya and receive the more advanced seed of the all-inclusive Name and Pastimes as Nam Shiksha, not as Nam Diksha. In this way, once can gradually advance and become an Uttama oneself even though one’s Diksha Guru is or was a Kanishtha.
Of course it is entirely possible that due to the presence of false ego, a Kanistha Guru may become envious towards such a more advanced disciple or another Guru outside his or her group just because of their advancement and not on the basis on any siddhanta. In that case, the Kanishtha Guru loses the qualification to be a Guru in the first place.
That said, in order to understand who is a real Uttama Adhikari, one needs to be at least a Madhyama Adhikari oneself. One can understand the advancement of a Guru based on the potency of the sound vibration or the written vani produced by him or her (mukha prasadad) and how deeply a Guru can answer philosophical questions. So these Kanishtha Adhikari Gurus play a big role by slowly raising themselves and their disciples to this Madhyama Adhikari platform, where they and their disciples can truly understand, find, and take shelter of an Uttama Adhikari Guru.
This is only possible though if these Kanishtha Adhikari Gurus put great emphasis on Nityananda Bhakti for Hare Krishna Rasa as Nityananda is the Samashti Guru Tattva; are very very careful to avoid hearing, reading, or speaking criticism of other devotees; are open-minded; non-sectarian; don’t think that pure sadhu-sanga is only limited to their own group, and don’t think that there is no one more advanced than them in this world. If they are against any of these principles, then these Kanishtha Adhikaris should never become Gurus in the first place.
Conclusion About Female-Bodied Diksha Gurus
If one is an Uttama Adhikari, then one is naturally a Jagadguru, whether one is in a male or female body. Jahnava Mata, Ishvari Thakurani and Hemlata Devi (wife and daughter of Srinivasa Acharya), Gangamata Goswamini, etc. are just a few examples of Mahabhagavata female Gurus who fall in this category.
A Madhyama Adhikari Guru is extremely beneficial for the whole world because Uttama Adhikari Gurus are very few and rarely found. So in that case also it does not matter if one is in a male or female body.
And because true Madhyama Adhikari Gurus are also quite rare in this world, Kanishtha Adhikari Gurus, if they very carefully follow the qualifications mentioned above, can bring great auspiciousness to this world by accepting and guiding disciples with full non-sectarian humility and awareness of their actual position. Thus in such a case also, it does not matter if a Kanishtha Adhikari Guru is in a male or female body, because eventually our soul is always inherently spiritually feminine or the enjoyed prakriti of the enjoyer Purusha Lord.
PS. Read here how sometimes strangely a Nityananda-nishth Kanishtha Guru may benefit the world even more than an Uttama Guru who is more Krishna-nishth externally due to his/her eternal sambandha-rasa?