It does boring and hardly even feels good. HE could be having sex with my hand, it doesn’t feel much different to me. Sometimes it feels good the first minute then goes away. Sometimes it’s uncomfortable and can’t hurt a little if he goes too deep
Some men have trouble maintaining an erection on their back. Google it you'll see. I don't know if there is a way to fix it.
If he is against you masterbating during sex he is being selfish. Tell him you're going to masterbate or you're going to find another partner. You're entitled to enjoy sex also.
Don't you think that it might be something psychological? You might need to see a doctor. You don't have to fake its your body and your temple