Thanks for putting it so bluntly. When you’re having sex with the same person over and over and over again, you still want to feel like you’re getting the full spectrum of sexual experiences. You know the Usher song lyrics, “Lady in the street but a freak in the bed?” Well, your girlfriend wants the same thing—just all in the bed.
The safety that comes from a good relationship often allows people to open up sexually in ways they might not with a more casual partner.
It could be that your girlfriend’s always had fantasies about getting straight nailed but that kind of sex with a more casual partner felt unappealing. But because she feels cared for and respected by you, she can enjoy getting fucked because deep down, she knows there’s nothing at all disrespectful about what’s going on.
And if she feels respected by you, she’ll want to express that in the sack as well—which is why at times she also wants to make love.
Keep switching it up!