Here is a short piece written recently by Ven Sumangala Mahanayaka Thera on bhikkhuni ordination, which he has kindly forwarded to me.
Ven Sumangala has been instrumental in introducing bhikkhuni ordination in Sri Lanka through the Dumbulla chapter of the Siam Nikaya. As his article suggests, this is also linked with his belief that ordination within caste, which is normal in the Siam Nikaya, is an innovation that has no support from the Vinaya. The Buddha frequently said that in entering the Sangha we are leaving behind all socially constructed notions of ‘class’ or ‘caste’, and saw the Sangha as quite separate from these constructions of inequality. In both cases we see the narrowing and increasing exclusivity of ordination in comparison with the radical renunciation of social norms that was envisaged by the Buddha.
I would especially point out the statement in the third paragraph of the second page, which very clearly states the notion of consensus as mandated in the Vinaya: the ordination should be carried out with the agreement of the Sangha in the sima, in accordance with Dhamma and Vinaya. Obtaining consent of Sangha elsewhere is not a necessity in Vinaya.
On the contrary, as Ajahn Brahm has pointed out to me, the Sangha is obliged to accept the validity of a sanghakamma that has been well carried out, in accordance with pacittiya 63:
Should any bhikkhu knowingly agitate for the reviving of an issue that has been rightfully dealt with, it is to be confessed.