I don't think it has to do with thinking with your d.., I think it's something else. I was feeling that too.. if I end up marrying my current boyfriend, I will never be with another man and considering I'm really young I think I would have wanted to enjoy my singlehood a bit more. I never thought about it this way, but it's been on the back of my head for some time.
Someone once said "When we date, we still have the urge to sleep with other people. It is the love for the other person that prevents us from doing it". I would never cheat, but maybe I do wish I'd met my boy a year from now.
What can you do? Monogamy is just too cruel of a rule. Hahahahaha I'm just kidding here But you get the idea.