Hi, dark_sunshine. It kinda sounds like you need to take some more time to really think about why
it is this makes you uncomfortable, especially when you're only making assumptions about his identity and don't know for sure. As Sam W said, wanting to try anal only means that someone wants to try anal. Nothing more, nothing less.
What is it about the mere idea of him having sex with men but not women that makes you uncomfortable? What is it about the mere idea of him having done anal in the past that makes you uncomfortable? What is it about the mere idea of having sex with someone who is bisexual that makes you uncomfortable?
You're clear on being uncomfortable but not on why
. I think you might need to dig a little deeper into your thinking because it does sound like some biphobia is at play here. If it is, no one here is condemning you. Many of us have had to unlearn messages like these! If not, cool - but what is it? It'll be a lot easier to address whatever it is that's making you uncomfortable here, if you're able to actually name it.
Have you watched Jane the Virgin? It's a drama/comedy based off the romanticism and drama of telenovella's (like soap opera's). I bring it up because your posts are reminding me of a scene from the show! In one of the later season's, Jane dates a man who comes out to her as bisexual. She doesn't have the greatest reaction and freaks out. This is the resulting conversation they have about what his bisexuality means for their relationship (basically... nothing - and definitely nothing bad!)
. Maybe I suggest you watch it? It's short, this clip is age appropriate, and you don't need any more context about the show to understand it. At the very least, I think it may give you something to think about. Maybe it will help you be able to better voice what you're concerned about?
New things are often strange and even uncomfortable for us. Maybe you just need some time to learn a little bit more about bisexuality to be comfortable with it?
All that said, I must really caution you against assuming your boyfriend's sexuality, though. We're all our own best judges for what our identity is. Don't get too caught up in labeling him as bisexual that you make the conversation about pushing him to come out as you see him that you forget to address what it actually is that's bothering you (that you're not comfortable doing anal - it sounds like - and that you're having some doubts about the relationship). Him ID'ing as bisexual or not isn't going to address these issues one way or another. Focus on what's in front of you, not making unfounded assumptions based on those facts. That's only going to be a distraction to you both!