Girllllll, you are way too tolerant. See my rule when dating guys was always, you can LOOK but you can't TOUCH. & I understand that it's human nature for beauty to catch our eye, and so when we see someone attractive we automatically want to stare. But as your boyfriend, he should know better than to bluntly stare at other women's asses right in front of you.
Even the most secure and confident woman will begin to have an altered sense of self and their own beauty if their man continued to do this. It's rude, point blank. It's like a form of seeing what else is out there, when you supposedly care about your woman. As women we naturally want to feel feminine, so when our boyfriends/ significant others do this it really makes us compare ourselves to other women. To be honest, I don't blame you for feeling that way. I remember my first boyfriend used to check out other women from head to toe, and I didn't want to say anything because we had just started dating. So I purposely started doing the same thing when I saw attractive men walking by, and I could easily tell it bothered him too. So he mentioned it once to me and said, "So hows his ass look?" and then I said, "almost as perfect as that women's boobs from an hour ago" and I remember we both just started laughing lmao. It brought us closer, tbh.He stopped doing it front of me because he knew how much it bothered me, and he knew how it felt.
I'm not telling you to do exactly as I did, hahah. But try to bring it up in a way where you don't victimize him, but where you really tell him how it makes you feel. Guys do have the ability to refrain themselves from looking. Just like we as humans all mentally program ourselves to not check out family members. It is by no means EASY, but nonetheless possible. I can enjoy looking at other attractive men too, but I never felt the need to because I liked the person I was with, and didn't out of respect as well. I bet if you started repeatedly checking out men in front of your boyfriend, he would start questioning his own physique as well. If he loves you, he will try to not do something that he knows really upsets you.
Good luck to you and your relationship! :)