Why Do People Have Sex In the Women's Room

Men because they'll take it anywhere they can, presumably. As for women:

But in the case where there’s a men’s and women’s restroom to choose from, heterosexual couples almost always go for the women’s room. “Women are much more apprehensive to go into the men’s room and have sex with a man,” Popovsky says. 

I do believe the DQ bathroom I had a dalliance in once was the men's room, thank you very much.