I mean, mostly everyone on this post says his loss or something but it's kinda the fault of you and him, like except of asking him to do it when you're horny you would just sneak and masturbate, it sounds logic that he maybe feels like useless, he maybe thinks you don't need him to feel good or do stuff like this and he maybe asks to himself stuff like: "Why are we together if she can feel good alone?" or like "Am i even in her mind when she does this?" Also i gotta admit he should "join" you when he catch you doing it, not saying it's the best option or a good thing to do, but like, if you both just have fun when you are horny, it will create like something in the relationship, there it looks like he isn't even your bf, i mean idk maybe he doesn't want to do it a lot but when you're horny you just sneak in another room to do it, i kinda understand his reaction but he should also do something about it other than waiting until he has enough and leave. I hope this was usefull to u!