Sometimes bad relationships go on because of good sex:

This is a somewhat common problem because being intimate with a partner creates attachment, not only for the women but for the men too. Since sex is a basic need, it’s fulfillment may seem more obvious or pressing than deeper emotional and intellectual needs. But all three are necessary for a full, significant love. If your relationship is based on simply lust, that is a warning sign for turbulence ahead. In a situation like this, it is hard to look at the situation from an outside, objective perspective because of the sexual attachment, but you must do so and when you do you will see the lack of significant connection. A relationship like this is by no means a bad thing, as long as you know what you want. A lot of my girlfriends get placed in these situations seemingly without their consent but their problem was getting involved with him in the first place. If you are looking for an exclusive hook-up buddy this may be sufficient for you; but don’t be in a relationship that isn’t working because of the sex, there’s more where that came from.