Everything I say here is going to take a hit, and it won't really matter what that is. But I like your framing here, so I feel it's worth responding to.
There is more to a relationship than sex. Just sleeping with someone else won't give him enough information.
Honestly, it's doubtful. It might help ease some of the concerns, but there will always be more. Nailing a willing stranger isn't much info. Sex takes time to practice with each new partner, and would likely be a bit off due to having learned one person really well. So even if you got that general wherewithall on how different people can be from each other, you might not be entirely satisfied. It can take a couple of rounds before you get a grip on how similar people are, and what kind of person is best suited for working together with you to create awesome stuff (relationship, sexytime, vacation, sunday afternoons, whatev)
get into another serious, long-term relationship to really be able to judge.
No, no I disagree with the 'long-term' part. You don't have to play out an entire melody to know it's going to be catchy or fall flat. You just have to gain enough exposure to know where it's headed, and make a call from there. Could be minutes, could be months.
I think it's very possible to have been each others only serious relationship and only sexual partner and still be very content and happy.
Anything is possible, so that isn't saying much. But is it likely? Stats say it isn't- most people will have between 5 and 7 partners before they slow their roll.
I also think you should take a closer look here at how you've qualified out with the word 'serious', meaning that even you don't believe it's very likely or possible having only 1 dating relationship ever. Think back to the kind of romantic relationships you were having at 16. Imagine that process was the sum total of your partner selection abilities. You gonna let that kid choose your life for you? I sure as fuck wouldn't- that kid was durmmm.
Being someone's first and only can absolutely work, but both partners have to be content with it, but that is not the case here.
Anything is possible, not everything is likely. I don't think it's reasonable to set that scenario up as the natural outcome. It's an outlier at best.