Often, but it never has been a problem. With all my boyfriends so far I've had the "cheat and be cheated" agreement, meaning we both allow each other to cheat. And it works! Well I got to admit, some guys couldn't handle it so I broke up with them, but most of them like it. I don't get mad when I catch my guy cheating, I grant him that pleasure. And he grants me the pleasure to cheat with another guy.
I've never cheated, I never caught a boyfriend cheating but when I was a teenager I caught my father cheating on my mother with her best friend. I was the one who told and once it came out my father left us for her, that devistated me and stayed with me for a very long time. My mother still harbors ill feelings towards my dad over that, but I did finally come to peace with it, I felt like I was the reason my dad left. But now 22 years later he is still with the women I caught him with, that relationship has lasted longer than his marraige did to my mom.