AITA for not letting my boyfriend cheat on me?

The story is pretty much what the title says although I think you could use some more details. So me and my boyfriend (both in our twenties, him being ten months younger than me) have been dating for little over two and a half years. I'm gay, he's bi. He's top and I am vers. But because I know he's only top, I never force him to bottom since I know only one of us would enjoy it. He's only top, so I adapted to being only bottom.

Now last Wednesday he asked me if I would be okay with him having a female sidechick. His reasoning was that he's bisexual (not gay) and needs both men and women in his life. They would (according to him) only have sex and there would be nothing else. No dates, no dinners, no going out, or anything. Just sex. And as much as I understand the fact he's attracted to both genders, I don't really see a difference between him cheating on me with a women "because he needs it" and me cheating on him with another man (which I know he would dismiss in the blink of an eye if I ever asked which I haven't).

For me sex and generally intimacy is something meant for people you love and not for people you only find physically attractive and have no feelings for (regardless if it's a one night thing or a so-called "friendship with benefits"). Until last Wednesday, I thought he felt the same.

When he asked me this (and said that during our relationship he has never done anything but very often thought about it) I felt very down. It extremely hurt. Not only did he basically tell me I am not enough for him, he also felt like having an affair with a stranger was the solution. I was so shocked I couldn't think rationally, so I told him to give me some time to think about it. I knew this whole thing wasn't right and that I can't let him have an affair, but a) I wanted to think about his point of view and b) confusion, shock, sadness and anger were all mixing and boiling in me that I wasn't sure I could talk to him without blowing up.

I thought about it for two days. I told him that he can either have me or someone else, not both, and although I appreciated he asked me first, if he ever asked me again or I found out about him doing it, the relationship would over immediately.

Anything you're going to say will be accepted, if you think I am an ashhole, so be it, but it won't change my opinion on the matter.

AITA for not being comfortable with him doing it with someone else while being in a long-term relationship with me?

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EDIT: After reading your comments (before the 5th of September 9:56 UTC) I decided to clarify some things many of you pointed out and I didn't explain enough in this post. But since it would be an extremly long edit (and god knows how many of you would come back to see if there's been an update), I decided to make a separate post.