Reason nobody tried sex with the girl

As you can see, all of these guys have been in the maze for a long time before Theresa appeared, and had already switched their orientation to homosexuality, so Theresa wasn't attractive to them anymore.

Thomas was a different case. Film hints he was already gay when he was sent to the maze. The romantic tension you see between him and Theresa is nothing but a red herring, you can clearly see that he shows as much affection (or even more) toward his male counterparts, specially Minho. This can be seen in Scorch Trials. When he was kissed or imagined to be, he freaked out. If you pay attention, he didn't seem uncomfortable simply because it was Brenda instead of Theresa, but because it was a chick.

I haven't read the book, so it might be different, but in the case of the film, it was obvious the writers wanted to take that turn, but were too afraid to make it more openly.