is he cheating on me???

[Blue]

To me it sounds like a communication break down.

"We're monogamous, but if we do mess around with anyone else, we'll tell each other." Translates to "It's okay with me if we cheat on each other as long as we tell, I won't get upset or break up with you."

That's an open relationship, that is permission to hook up with someone else if either of you wants to. That doesn't mean ask me if you can, it means go on and do it, then tell me whenever you get around to it.

Whether you see it that way or not, you gave him permission to hook up with others, so he can't possibly have cheated on you. You can't cheat on him either, since you also gave yourself the same permission.

Now if you had said "Do not cheat on me but, if you slip, I'd rather you tell me than to hear it second hand." That's different. then you are simply saying you don't want him to cheat, but if he slips, you'd rather he tell you than have someone else tell you.

You two need to have a talk and get things straightened out, then decide what kind of relationship you want, if any, with each other.