This is a difficult one to navigate.
Personally, I am on my second relationship and I don't feel the need to be with another person ever again for the rest of my life. That's just how I feel because nothing would ever match up to what I have right now, I'm not missing out on anything.
I believe this to be a combination of already knowing that sex and stuff is really only fun for me with someone I love, and that we are just so right for each other that I don't need to see what else is out there.
Can I ask, do you feel like the reason why you feel this way is just not knowing or being inexperienced? Or do you think you're likely to find something better than you have now?
In my experience "you just know" when you're with someone you want to remain with. It's just one of those things that I think we have gut feelings about. Do you feel excited and happy about the prospect of living together, getting married, having kids and basically being with this person forever? Or is it tainted by sadness that you'll have missed out?
I could assure you that if he's the one for you then you're not missing out, but the fact you're feeling like this makes me wonder if he really is the one for you.
What I will say is that if you love him and don't want to break up with him, then don't. Enjoy your time together, look after each other. Nobody is talking about marriage yet are they? but if you can't continue on without feeling like you want to be with other people, don't lead him on if he appears to want to get serious.
Listen to your gut - it isn't wrong.