There's actually something to this theory: I think it is likely that feminist women are not as attractive as non-feminist women.
I was inspired to think about this theory by a married female friend of mine who is rather attractive. She proclaimed that she was not a feminist, and that attractive women would not be feminists because they benefit from 'patriarchy'.
Cui bono? (Who benefits?)
I studied a bit of sociology a long time ago, and one of the theories we learned was Conflict Theory (CW Mills, in the tradition of Karl Marx). Basically, in analysing and critiquing a social system, we look at who it benefits and who it disadvantages. Those who benefit are thus those with a vested interest in maintaining the existing system, and those who are disadvantaged are those who advocate its change (or dismantling).
Example of conflict theory analysis
Returning to the question of feminism and female attractiveness, feminists proclaim that (among other things) they are against 'patriarchy' because women are only valued by their looks. Immediately, we can see that (if this is true) attractive women have a vested interest in keeping 'patriarchy' and unattractive women would like to topple it (so that they can move up the social hierarchy).
Inasmuch as people hew to ideological positions that personally benefit them, we can conclude that feminist women indeed are not as attractive as non-feminist women.