Christians: could you explain this?

Adultery is USING another person for your own personal gain. Any sort of sex without a love commitment to the other is simply treating the other person as an object which exists just to satisfy you. You are in a sense treating the other like a inflatable sex doll. A thing. The fact that there might be mutual consent means nothing... that is just two people using the other. It abases not only the other person but yourself as well.