Why my guy friends make sex jokes that include me?

hey! My group of friends includes guys and girls, some of them are single but some of them are taken. I don't like anyone and I am 100% sure that no one like me either. I am a shy girl, just like the rest of the girls, generally I don't talk too much, I am pretty much mature (I think) and I am not nothing special.. I don't talk about sex or my personal life to my friends. Anyway, there are two of my friends (the one is taken - his is in a relationship for the past 5 years), that always make fun of me, and they always making jokes about having sex with me! I am not mad at them, I know that they are not serious and I know that this is just a joke. I find it funny, I always laugh, I don't really mind , and I definitely don't think that those two friends like me. I am just wondering (pure curiosity) why my guy friends make sex jokes including me and only me and don't do it with the rest of the girls? OK some of them are taken indeed but still! they don't make jokes with the one who are single.. is it my fault? why do you think that this is happening?
Why my guy friends make sex jokes that include me?
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