Having sex with someone else is just as much cheating for men as knocking one out to some random internet porn.
It's when you're brain gets involved that you're actually making love.
With girls the brain gets involved every time (cos it's more of a mental thing with them whereas it's a physical thing with blokes) and therefore they cheat every time whereas blokes only do sometimes.
or something like that.
(sigh, the justification always sounds so good in my head
)