Question for those in long term relationships who have only had sex with that person

My spouse and I are pretty young. We’ve only been with each other, sexually. And that’s completely okay for both of us. For now. I’m afraid someday he will regret having not experimented with other people. So have any of you or your partners felt that way? Do you have this fear? We have a great sex life and are very sexually compatible. So it really isn’t an issue right now. I’m just apprehensive for the future and could use some guidance.