Bad sex is always a disappointment, particularly if it’s supposed to be a casual sex relationship where you’re using each other for physical pleasure and comfort. Bad sex is more tolerable in a long term relationship that gives you great emotional comfort (although even there it shouldn’t be tolerated), but when it comes to something more casual it damn well better be good sex or there’s no point. You have a few options if you’re experiencing bad sex. One is to end the relationship immediately and another is to drop a few hints and see if you can change things up.
The worst thing you can do is tell the girl she sucks at sex. Sitting her down to let her know that the sex is bad and that she needs to do something about it is guaranteed to cause a fight, hurt her in ways you can’t imagine, and most likely end the relationship. Plus, it’s really mean and you don’t want to be the mean-spirited guy. Instead you need to be subtle about it.
Illness of some sort is one of the most common excuses a woman will throw at you when she’s not in the mood. There’s no reason you can’t do the same thing. Plus, in a man’s case it’s well established that this sort of thing is exceedingly rare, which ensures your message will get through to her. Do it enough and she’ll know that something is up and will eventually get around to wondering if the sex is bad. You need her to get to the place on her own. If she brings it up then you can talk about it without being insulting.
If you want a more direct approach (though not fully direct) ask her if there’s something you can be doing to make the sex better for her. Betray no hints of selfishness when you do so. Instead, make it perfectly clear that you’re only asking because you want to make her happier. If she deflects the question by asking if there’s something she can do for you, bounce it right back at her by saying that you’re asking for her pleasure and you’re not concerned about yourself at the moment. Only after she’s detailed how you can make the sex better for her is it okay to get into how she can turn the bad sex into good for you (without mentioning that it’s bad sex).
It’s a little aggressive, but getting caught masturbating or making sure she knows you’re masturbating is a good way of getting her to question her performance in the bedroom. She will likely not understand why you’re jerking off instead of having sex with her and she’ll either ask you about it, at which point you can let her know about the bad sex and how it can be changed, or she’ll try harder in bed. If you really want to be down low and a little mean in the name of better casual sex you can suggest that she’s getting a little heavy or casually hint at great sexual partners in the past that obviously pleased you more than she did. Remember, a casual sex relationship is not one where you need to be constantly concerned about the emotional well-being of your partner, which is why you can embrace these underhanded tactics.