I see that in some theravdan traditions woman cannot fully ordain (some thai traditions). But in some burmese and tibetan traditions they can. From what I understand it is because of the break in lineage but how do these other traditions justify it?
I wasn't aware of any Burmese traditions allowing for female ordination; in Burma it has always been seen as a terrible heresy, afaik, with one bhikkhuni actually being thrown in jail.
The gist of the main argument against goes as follows:
premise: whatever it says in the vinaya pitaka has to be followed to the letter.
premise: the vinaya pitaka says that female bhikkhuni ordination requires both bhikkhunis and bhikkhus to succeed
premise: the bhikkhuni lineage is extinct.
conclusion: bhikkhuni ordination is impossible. Q.E.D.
The proponents of bhikkhuni ordination challenge each of the three above premises in various ways, as follows:
They advocate breaking certain rules in the bhikkhuni vinaya by ordaining more bhikkhunis than allowed, etc., hence they challenge the idea that the vinaya must be followed to the letter.
They deny that the vinaya pitaka requires bhikkhunis in order to ordain bhikkhunis, hence they claim there is no textual backing to the argument against ordination of bhikkhunis by bhikkhus only.
They deny that the bhikkhuni lineage is extinct, citing existing bhikkhunis in Taiwan, I think.
I dislike the first of these challenges; it seems dangerous to pick and choose rules, and the broken rules have more to do with the specifics of ordination, rather than the actual ordination itself.
The second challenge is what finally turned me around to supporting bhikkhuni ordination; there really is nowhere in the vinaya pitika where bhikkhunis are explicitly required for ordination. Here's how it actually goes:
1. The Buddha tells the first potential bhikkhuni that in order to ordain she would have to agree to the rule (among others) that:
When, as a probationer, a bhikkhuni has trained in the six rules [ cha dhamma ] for two years, she should seek higher ordination from both orders.
2. When other applicants arise, the Buddha says:
Bhikkhus, I allow bhikkhunis to be ordained by bhikkhus.
3. When applicants for bhikkhuni ordination become too shy to answer questions posed by bhikkhus about their physical health, the Buddha says:
Bhikkhus, I allow for one who has been ordained to one side, found to be pure in front of the bhikkhuni sangha, to ordain in front of the bhikkhu sangha.
The question is whether this new allowance supersedes the first allowance. I don't think the original rule agreed upon by Mahapajapati can be taken as law, since it was subsequently broken with the first allowance. Further, the eight rules are worded to be simply a warning of the severity of the rules they would have to follow. Finally, the emphasis in the one quoted above is on the two year probation, not the dual ordination.
As to precedence in regards to alternative allowances, we find two distinct patterns in the vinaya pitika:
Where a certain allowance is given, then rescinded and a new allowance given.
Where a certain allowance is given, then a new allowance is given without rescinding the old allowance.
Examples of the latter include first allowing clear broth for a sick monk, then allowing thick broth later when the clear broth didn't solve the problem. This seems analogous to the bhikkhuni ordination, in that both cases were brought about not by some danger in the original allowance, but by circumstances where the first allowance was insufficient. It seems therefore reasonable to suggest that just as there is no reason to forbid clear broth now that thick broth is allowed, there doesn't either seem to be any good reason for forbidding bhikkhu-only ordination of bhikkhunis just because dual ordination is now allowed (remembering that it was enacted for the sole reason of allowing shy applicants to answer embarrassing questions)
More telling, perhaps, is the best example of the former pattern, where the original allowance is rescinded: the case of bhikkhu ordination. Originally, the Buddha allowed bhikkhus to ordain under him by saying ehi bhikkhu - "come bhikkhu". Once it became apparent that this wouldn't be sufficient to meet the demand, he allowed other monks to give ordination using the tisaranagamana (going to three refuges).
Eventually, it became apparent that greater oversight would be required in the ordination process - i.e. there was an inherent flaw or danger in single bhikkhus ordaining bhikkhus. So, he says:
yā sā, bhikkhave, mayā tīhi saraṇagamanehi upasampadā anuññātā, taṃ ajjatagge paṭikkhipāmi. anujānāmi, bhikkhave, ñatticatutthena kammena upasampādetuṃ.
Bhikkhus, that tisaranagamana ordination which was allowed by me, that I revoke from this day forward. Bhikkhus, I allow one to be ordained by an act with the announcement as it's fourth.
Meaning, a quorum of monks had to hear the announcement and three declarations, giving them all a chance to reject the ordination.
The key passage here, missing from the second bhikkhuni ordination allowance, is in bold above. The fact that it is missing makes the bhikkhuni case categorically different from the bhikkhu case, and no textual based argument can suggest otherwise.
tl;dr: the Buddha never revoked the permission for bhikkhus to ordain bhikkhunis, therefore bhikkhus can still ordain bhikkhunis. Q.E.D.
As for the third challenge above (that there are still bhikkhunis in an unbroken lineage), I'm somewhat non-committal. They bhikkhunis in question are certainly not Theravada, so it's a bit of a stretch to suggest they "count". But there are arguments in favour of allowing them to "convert" to Theravada in order to perform the ordination. The best argument against this challenge is that, like the first challenge, it is unlikely to be accepted by the Theravada at large.
The issue is that the Theravada vinaya instructs that Bhikkhunis (Fully ordained Nuns) must be ordained by other Nuns and then the ordination is ratified by an act of a group of Monks. When the lineage of Nuns ended then (or so it seemed) there would be no way to ordain Bhikkhunis.
At least that's the argument that some Theravadins give. In reality, the Theravada line of ordination for Bhikkhunis never totally died out, but was transplanted to China where it was maintained as a lineage of Mahayana Nuns following the Theravada Vinaya. In 1997, this ordination lineage was transplanted back into the Theravada school making fully ordained Bhikkhunis a possibility again.
There has also been a theory laid out by the Ven. Bhikkhu Bodhi, the great translator, that because the original form of Bhikkhuni ordination was done by Monks and was never formally abolished, the Bhikkhuni order could also simply be restored by having Monks ordain the Nuns if there were no Bhikkhunis.
In East Asian Buddhism, they follow (with the exception of most Monks in Japan and some in Korea who follow a non-Vinaya form of ordination) the Dharmaguptaka Vinaya and lineage, and the Dharmaguptaka order of Bhikkhunis never died out. As for Tibetan Buddhism, I think they just used the Dharmaguptaka line of Bhikkhunis to restore it but I'm not sure.