there is never a valid reason to cheat, that would defeat the definition of cheating.
if you want more than one woman, tough luck, because once you get that reputation the only women you'll be getting are... well, prostitutes... and we wouldn't want to waste cash in this crap economy.
my advice to you? if you're in a relationship that you like, stay in it. if you want to cheat, end it, and get with that girl. but NO cheating. the consequences aren't worth it in the end. and if you're having second thoughts like that about the relationship you're in, you're just not worth that girls time!!
no one deserves that, i doubt you'd appreciate it much, either.