Well, at the heart of it, it is an advice website where people ask anything. People are naturally going to want advice for things they don't really know about, which often coincides with things that they can't really ask others about (out of shame, embarrassment, you name it).
Sex is one of those topics. Same thing with relationships. People don't know who to ask about the stuff so they go on GAG to ask it.
Granted, if you are actually referring to the pervy questions that are just people trying to 'get off,' that is just the downside of internet. It is best to leave those alone and pretend they never existed lol
I joined because I like giving advice and I think I am halfway decent at it. For the personal motivation, it actually makes me feel better when I get a "thank you" from someone struggling or whatever and I help them out. I joined after going on Yahoo Answers for years with that motivation, then realized GAG was (and still is) a better site.
When it comes to interaction between men and women, sex is an inevitable topic. Thankfully sex is not cut and dry, right or wrong topic. It inspires many questions and discussions. What pleases my partner most? How can it be better for myself? And everything in between.