SO doesn't want to have sex anymore...:(

So this has happened 3 times now. My SO and I plan to have sex and when I go up to bed he doesn't come up (like he usually would) and he chooses to jerk off instead - literally while I am upstairs, in the bed, available. I am so hurt by it and he knows it. I told him the last time I caught him how upset I was, I actually got so mad I "lost" it a little. He says it's because he know's I am tired but that is not true at all. I mention sex and then say I'm going upstairs like hello, get the hint idiot. I'm becoming so frustrated and sad, I feel fat, ugly and undesirable now. Just needed to rant.