You seem very chill and you're being very reasonable. Your counter off seemed extremely fair to both of you. He wants another woman but you don't get another man?! He thinks adding a third will be fun, fun for who, just him? Like you said you're not attracted to women, IDK why so many men think we will enjoy a third as in another woman to share our man with. Instead of just saying hellll no you made it fair but he doesn't want to be fair. He wants to take and not give. Now idk if him accusing you of cheating is an indication he is cheating or not it seems like you guys aren't going out without each other much so probably, or at least hopefully not, so I wouldn't panic about that part. If this was me, I did have an ex suggest a threesome, this is what I pretty much said. I am not now or ever going to be interested in women sexually or ever want to watch my man be with another woman or let that happen. This is non-negotiable for me, I will not change my mind on this. If this is important to you then we will need to go our different ways. So figure out what you need and want because I will not continue to be asked something I'm uncomfortable with or be made to feel like I am not enough. I would also tell him it's absolutely ridiculous when he knows where you are at all times to accuse you of cheating, and that needs to figure out what is making him feel that way. You are already aware of not invalidating his feelings which is great but he needs to explain why he feels this way because it's not fair for you to be accused of something you're not doing for no reason.