Second wave onwards. Third wave didn't appear out of nowhere. All the elements required for intersectionality (e.g. postmodernism, social constructionism, and the oppressor/oppressed worldview derived from Marxist thought) were there in second wave feminism. In the context of second wave these were used to describe how women are perpetually oppressed by males through societal construction of gender and gender roles. If women truly can do everything then it follows that so can men, and some men became women.

Not all feminists subscribe to intersectionality, although it's quite noticeable that dissenters, previously content with the change, seem to have increased in numbers when the ideological view led to a small percentage of primarily white heterosexual men finding their way to the top of the progressive stack. It was at this point the implications became rather clear, with lesbians who refuse to have sex with transgender women becoming bigots and the male invasion of previously female-only spaces.