1. Transwomen are women.
2. Title IX does not govern individual membership policies. A sorority could welcome transwomen and still bar men from joining without any legal repercussion. Title IX allows schools to recognize social fraternities and sororities regardless of individual gender policies, as opposed to other types of GLOs, which are not exempt.
3. Social fraternities and sororities could welcome opposite-gender people if they wanted to and still not lose their right to determine their own membership. I know a woman who pledged an NIC fraternity as a male freshman and then transitioned to being a woman. As far as I know, she still maintains her membership.
Anyway, good for Brown!
https://www.campuspride.org/wp-conte...DOE-Letter.pdf
https://www.campuspride.org/wp-conte...s-Title-IX.pdf