My boyfriends bedroom habits bug me

Please keep an open mind! I really need advice…So I’ve been with my on-again, off-again boyfriend for almost 5 years, and our sex life has always amazing! We are currently off at this time, but the make up sex and sex in general has kept me going back time after time! But theres ONE thing that I just don’t know if I can get over, accept, or even enjoy! It’s hard for me to even say this, but he loves it in the backside or any kind of play in that area on him. Which is typical for a lot of straight men, yes, but he can actually take a bigger toy than I can in the back!!! And I’m just not comfortable with knowing my man is that into that, To me, it takes away from his masculinity and all around manhood. But, he’s open with me (After 4 years) about how much he enjoys it, and can only go when his prostate is played with, so just normal sex like we used to have is out, buf Maybe my feelings are wrong, but I cannot seem to fully enjoy watching him do that to himself, me doing it to him, or look at him the same really, but I DO NOT HAVE THE HEART TO TELL HIM THIS, bc he trusted me enough and was open enough, after 4 years of being together, to tell me how much he liked this and enjoys it. What do I do??!?! At times, it’s hot, and at times I’m totally turned off and freaked out! I know this a weird situation to be reading about, but I honestly cannot ask my friends, LOL…does this make him gay or bisexual? Or does he just like this??? Please don’t be immature and give me some open minded, mature, advice. This is my only way of knowing if I’m right or wrong by feeling the way I do! My total outlook on him changed when he told me the first time, I told him I didn’t feel comfortable with him doing this but over time he has talked me into it so much that I kinda just let it go. But, it’s not something I can just overlook I don’t think! He wants to get married, we both have kids, ( not together) and they love each other, our families get along great, my friends love him and his kids, and he’s an all around, nice guy! But this is just the one thing that I seem to struggle with about him! HELP!!!