If premarital sex is so bad, why does it feel so good?

We cannot determine the goodness of an action based upon how good it feels. Rapists and child molesters feel pleasure, but no one would doubt that their actions are immoral. Likewise, things that cause tremendous pain can be acts of great love, as when one person gives his life for someone else. If we measured the goodness of an act by the pleasure received, then adulterers would be virtuous and war heroes would be scoundrels.