assuming a committed relationship:
if you love your woman, don't cheat.
if you don't love her, then don't be in the relationship.
if she does cheat, and it bothers you, question whether you should be in the relationship. if it doesn't bother you...then, ditto.
i dunno why i'm responding to this, except it seems simple (on the surface) compared to some things people may deal with soon.
of course, these things are never simple really. i know this.
best...
ps: if you are dealing with a sociopath or psychopath (or someone who professes to not understand your sense of morality, and you instinctively care about this) run don't walk. you won't regret it. s/he is not right for you. BTDT.