Sex is definitely a skill, and like any skill, it is refined with education and practice. Humans have a natural sexual desire, but unlike bugs and rodents, there isn't a prescribed way to do it. There is a high educational curve. The more you read and the more you experience it, the more you will know. That is a wonderful question for which there is no concrete answer.
Therapists are divided on what constitutes cheating.
Love has to be painful because it is so deep. You are making an emotional, physical commitment to another person that innately empowers them to hurt you even if they don't mean it. So love, joy and pain sort of all come together. You will have to trust that your lover will only want to give you joy. So your task is to ask him what happened.