If you are his only and he is your only. Curiosity just gets the better of either one you (most likely the man) after so long. Could be a month or 10 years. Being with 1 sexual partner is just not in the cards.
Me and my husband both had enough sexual partners before we got together.
I think you should sew your oats before tying down to one.
Now some people on the other hand, will never stop. They will just screw anyone and everyone forever even if they are in a relationship.