I'm not sure that it's ever appropriate for a girl to cheat, but here are some guilt free ways men can use:

1. The Row

Engineer a fight and cheat that same day, or the next day, or the day after that.

2. The Holiday

Feeling deserted is a good reason to seek comfort elsewhere.

3. The Cure for Impotence

If you can't get it up, or think that you might suffer from erectile dysfunction at some point during your life, it is acceptable to practice sex on a prostitute so as not to risk hurting your birds feelings should your erection fail.

4. Practice

It's OK to practice sex outside of a relationship if it's to learn new tricks to improve your partner's pleasure in bed.

Hope this helps.