I always assumed Tosh was bi, based on the "Day One" scene combined with the general bi-friendliness already established via Jack and Owen. GBG confirmed it for me; she was uncomfortable with being the center of anyone's attention, but unsurprised and unconcerned about it being a woman's attention, and her line about her parents struck me as someone who's spent some time thinking about her parents' reactions to her sexuality before.

I didn't see anything in GBG that would make me think that was her first time with a woman, nor did I see anything that would suggest Mary was manipulating her into something she didn't want to do anyway, much less actively controlling or telepathically influencing her. The pendant allowed them to read minds; it was never suggested that it allowed them to influence another's thoughts. Yes, Mary took advantage of Tosh's vulnerability, but that vulnerability was pre-existing and could have been taken advantage of the same way by any smarmy human guy; Mary's femaleness was irrelevant, and her alienness was relevant only as the motive for targeting her. A human with a different motive could have done the same.

I will agree that it was "generally not the sort of thing Tosh would normally do", hence her discomfort during the bedroom scene, but I always read the "it" there as being "sleeping with someone you've only just met", not sleeping with a woman.