HELP!!........I sometimes get upset when my boyfriend masturbates instead of having sex with me.

Yes, obviously he's a man and he has needs and I'm fine with that. In fact I masturbate quite often myself. The issue is that lately we've been having alot less sex and because of that I feel slightly offended when he doesn't ask if I would like to have sex and instead just jerks off. He's actually told me he sometimes prefers to jerk off then to have sex with me. It just makes me feel like I'm doing something wrong because we used to fuck 5 times a week, now I'm lucky if it's 5 times a month. I've tried talking to him about it but he doesn't really seem to grasp just how much it affects me. Ladies, am I over reacting? Or is there a cause for concern here?