Sex wouldn't be good for me if that's all we had. I need to be emotionally connected to someone to continue having sex with them. But I gave been in a terrible, awful relationship. The sex wasn't good, but we thought we loved each other. The bad was one-sided; my side. So I eventually had enough and ended it. But I should have months prior. There's no explanation other than love. And that doesn't even explain anything. It makes no sense.
Now, obviously, people stay in bad relationships for their kids, for the money or because it's easy. None of which serve as worthy reasons, though. If it's terrible, get out of it. It's that simple, but it's not simple to do.