Why Is Sex Good After A Fight?

There are a couple of reasons for this, though one is a much larger piece of the puzzle.

Firstly, after there is a huge conflict between two people where they are both pulling out all the stops, there is a change in their physiological state. Their blood flow has increased, there may be some muscular contraction, increased heart rate, central nervous system stimulation, etc. This sort of reaction is very similar to what your body goes through during a sexual act, so the gap isn't too hard to jump, just in a physical sense.

More importantly, after a disagreement, there is an emotional chasm created as well as a certain loss of the closeness between you and your partner. The make-up sex you have is a physical demonstration of reconciliation between the two of you. It's also how you rediscover that which is temporarily lost in your partner during the emotional distance you are experiencing.

This is all totally normal and widely reported, but you have to be careful that this does not become too frequent in your relationships. If the closeness you feel with your partner primarily comes from after you have a recognized emotional separation, it can be risky and potentially damaging. You don't want to be in a situation where one or both of you love to argue and fight just because of how you eventually feel at the end of the process. Destroying something just for the sake of rebuilding it is a poor idea.