Jeez Audiate, the thing is there was no reason to think she was straight either, aside from the higher probality of that being true, her being bisexual or lesbian isn't really that difficult to concieve of.
Yes, there was no actual reason that she had to be gay and yes, sexism did most likely factor into their decision to do that, but it still does not mean that there is really anything wrong in terms of the plot with her swinging the other way there.
I do understand why you would feel disdain for such a brazen display of sexuality, but I don't think it was misplaced or anything. They were in a brothel and it just so happened that she wanted to participate too. Is that really such a big problem for her character or the story?