Sure, things change, and people drift apart. But you know if you're in a committed relationship where monogamy is expected. If you want to sleep with other people, you should have that conversation with your partner because you're changing the nature of your commitment. Cheating isn't imperfection, it is betrayal. It takes effort to find an affair, to find the time to get together, to deceive the person whose trust you're breaking. There's no greater failure of character.