I feel like this is the best place to ask this so I'm just going to go for it. I'm still processing my own feelings about what I want and think maybe sharing on a forum will help. I've been seeing this guy (N) for about 5 months now, and he said from the start he "didn't want anything serious" "it's just casual" etc. and I was in the same boat - not looking for a "relationship". I was (and still am) also still in contact with a guy I'd met last year whilst travelling (J) who lives in Canada (we're in UK). We've talked before about how we like what we've got - sex is good, and we're not holding out that it NEEDS to be going anywhere, and have talked a lot about monogamy and how I think it's not for everyone and that we get different things from different people and he agrees. His last (and only) official relationship ended because he cheated a year in, and then again a year later and I said that knowing that I'm not going to try and force him into anything like that.

So now 5 months in I've developed feelings for him and I believe he feels a similar way, but isn't very emotionally expressive, and kind of want to broach the subject of actually saying we're "in a relationship" but still keeping it open. I mean, currently we are essentially in that, but just without the label, and I think calling it that would mostly be out of convenience - to be able to say he's my boyfriend. However, I'm worried that saying that will make him back off because he seems to think that a relationship can only end in either heartbreak or marriage and nothing in between, so just by giving it a name he might feel I'm holding out for long-term commitment, even though I'd still be up for openness in the relationship so long as there's communication.

I'm also not sure quite how OK he is with me being with other people (or other men at least), despite trying to "keep it casual". I addressed the fact that I was still in contact with J and that he wanted to fly me over there and N said he was understood it, but seemed a little uncertain in saying that. He also said that he might not be OK with it if the other guy was here in London, but that he had "said I didn't want commitment and I can't stop you if that's what you want to do". I kind of feel that he's not actually completely OK with it, but it trying to be because he doesn't want to go back on what he's said at the start.

Not really sure what I'm asking for here, but any advice on what I might say?

TL;DR: Currently partner doesn't want "relationship" because thinks they're constraining. What are best ways to suggest an "official" open relationship?