Why does my friends with benefits do this?

I have been friends with benefits with this girl for a couple of years now. She originally wanted a relationship but I explained that I don't. She said that she only has sex when she is in a serious relationship which is fine with me. We have been going down on each other probably once every two weeks. We talk about all of our life problems but we don't date. Recently she has been having sex with guys in her life pretty casually but she still refuses to have sex with me but I still get weekly bjs. She is always talking about how she fights with other guys and then has sex with them. She seems to be comfortable with me and I always treat her well but she won't have sex with me; which I want to switch to because her bjs are boring. When I ask her why she gives a dumb excuse like "women have to be careful who they sleep with or they get labeled a slut" (but you are sleeping with other guys you've known for a week so?) Also she is friends with my sister which might play into it. Is she trying to get me into a relationship with her in exchange for sex? That seems like it but I don't want that since she is not my relationship type. Her job sucks and she has no ambition. Plus if she thinks she can control me by withholding sex I am a little repulsed by that idea.
Why does my friends with benefits do this?
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