Infidelity Is A Cunt
To me, infidelity is a power thing. When my ex boyfriend slept with a fat, brash slag, I wasn't concerned that he'd seen her naked or penetrated her gaping cunt. I was more irritated by the fact that she'd had him. She'd met me, and fucked my boyfriend. She played on being everything I wasn't (at the time), everything he told me not to be. And she won.
Good job I screwed him over equally a year later after giving him absolute hell for what he did and making him wish he'd never been born.
Anyway, it's a matter of pride. And of trust. Porn stars can sleep around if they like and not give a shit who else their fellow-porn-star partner shags. Good on them for having the balls to believe their partner won't actually decide that hey, so-and-so's got a bigger cock than you, I'm going to leave you for him.
This is the inevitable flaw with infidelity. It's not always just about sex. You can have a realm of affairs that are purely physical, then fall for one person and everything changes. You can't help who you fall in love with. And, although I'm not a 'girlie girl', sometimes you do fall in love with people you sleep with.