Psychologically speaking, you're individual identity is stronger than your relationship and the ego searches for affirmation in the same way a Mcdonalds burger tastes 100x better if you're on day 5 of your diet.
You just don't do it, for a reason that merits it. If you have found a reason important enough to not, than you're set for life. If not, you should do whatever makes you happiest.
Even if something's wrong, if it's in your nature to do it, you should. Immediately. So that you learn enough,in your youth, that you want to make a better decision.
Doing the right thing without knowing why leads to repression, and you just make the same mistake later when it will cause more damage.