I’ll start by saying “If it works for you both, then it works” but this kinda rubs me the wrong way. To me it says, “I am entitled to your body but no-one else is. That’s what I primarily am interested in about you. If someone else gets your caring, your attention, your deepest thoughts, your emotional intimacy — I could care less because those things aren’t important to me.” Maybe I’m way off base here, but that’s what it telegraphs to the me. It would make me really pissed off if my husband said this to me.
When my husband and I opened up our relationship, we were intending for it to just be physcial and then along the way we met people that we had feelings for. This happened for me first, so although he was totally fine with me having sex with someone else, it took much more of an adjustment to deal with the fact that I loved someone else besides him. I think this is much more the norm.
Who knows what is really up with your guy — whether he’s just deluding himself or this is actually his comfort zone but I’d be curious to ask him why he draws the line where he does. From my perspective, no-one is entitled to anything of yours or has “dibs” on it. If you choose to share some part of yourself with someone, that’s your choice, and although you might make certain agreements with intimate partners, you cannot actually legislate love and commitment and your body is yours and yours alone (even if you decide to only share it with one other person). Fences aren’t what actually builds good relationships. Good communication, trust and deep intimacy are what builds good relationships.
This guy writes a lot of good stuff on this topic.