No matter what you tell yourself,if you cheat, you're a pos. If you can't be faithful, tell her, if she has a problem with it leave. Don't sneak around behind her back and put her health at risk because you want to be a disgusting pig. Just stay single and let her find someone that will respect her and treat her right.
and that's EXACTLY what it means, you can't be a "good guy" AND cheat. Why men try to convince themselves that they can treat people badly and still be considered a good guy, I'll never understand.