A man who is sexually attracted to other men can join.
A woman who is sexually attracted to other women can join.
A man who believes he is not a man can join.
A woman who believes she is not a woman can join.
Someone who sometimes believes they are a man and sometimes believes they are a woman can join.
In fact, anyone whose minds are at war with their body's physiology in regards to gender or sexuality can join.
There is only one type of person who cannot join, and that is someone who believes they are the gender their body implies, and who is sexually attracted to the gender that their own sex implies they should be in the context of reproduction and nature.