Vinaya has this allowance;
Now at that time the features of a woman appeared on a certain monk. They told the Blessed One about this matter. [He said,] “Monks, I allow the same teacher, the same ordination, the same rainy seasons together with the nuns. I allow reinstatement among the nuns for those offenses that nuns share in common with monks. According to those offenses of monks that are not shared in common with nuns, there is no offense.
Now at that time, the features of a man appeared on a certain nun. They told the Blessed One about this matter. [He said,] “Monks, I allow the same teacher, the same ordination, the same rainy seasons in relation to the monks. I allow reinstatement among the monks for those offenses that monks share in common with the nuns. According to those offences of nuns that not shared in common with monks, there is no offense” (Vin III.35).
I am not an expert on this but afaik it’s more or less like this;
According to a text included in the Canon, Buddha was hestitant in allowing Bhikkhuni ordination, he did however allow it and imposed 8 particular restraints.
At that time there were no Bhikkhunis and he allowed Bhikkhus to ordain women as to start the nun-order.
After this, as far as we can establish ordinations were preformed exclusively by their respective gender as unilateral ordinations.
Nowadays most monks are trained to think that the allowance for monks to ordain women (aka bilateral ordination) was rescinded once the Nun’s order was established, that nowadays only the unilateral ordinations are permitted.
Many think that women won’t have any teachers and that they will be ‘out of control’ and somehow damage something.
Some nuns and supporters seem to dislike the text which imposed the 8 particular restraints and permitted the female ordination in the first place. Some people have tried to make a case for it being a ‘fake’ but not much has come out of it.
Either way, not following the 8 restraints and trying to dismiss the canonical text seems to only antagonize the opposition.
There is also a case being made based on the initial hestitancy of The Buddha in allowing females to get full ordination, it is said that it made the Dhamma not last as long as it could’ve.
Because of this people don’t like the idea of a ‘revival’ of a bhikkhunisangha thinking that it will further accelerate the disappearance of good things.
I personally think that the ordinations are probably valid, in part because mn142 says that in the future there will be immoral “members of the clan” or “members of the spiritual family” [who will be ‘yellow-necks’ or ‘wearing yellow scarf around their neck’] and that gifts to them will be given on account of the sangha.
As it’s not saying ‘in future there will be immoral bhikkhus’ which would imo be natural if the sangha had at that time become unilateral, i think it’s likely that both orders are around at the time of yellownecks.
I also think that even if female ordinations weren’t valid they can still be said to be having gone forth into homelessness out of dedication to The Blessed One, be allowed to keep as many precepts as they like and be called Bhinkhunis because Pukkusati was called a bhikkhu by The Blessed One and was said to have gone forth in these terms without ever receiving a full ordination. See mn140
And I hope you have a bit more understanding and peace now 
That sutta has been an inspiration for my own practice 
