Some men and women can be trusted to have friends of the opposite sex and some can’t.
Exactly, as a red blooded male I’d happily fall into a pit of mindless fornication with any of the attractive women I meet on a daily basis if I was single, if they were single, and if both of us wanted it. But if I’m in a relationship or if they’re in a relationship then it’s a no go.
Doesn’t stop me wanting to have sex with them, it just means that I value my relationship with my GF and the trust within it over and above me wanting to get a quick bit of ‘leg over’. So I keep these thoughts to myself, treat attractive ladies with respect and don’t do anything inappropriate.
So if you’re in a relationship, provided your GF does not look like Shrek, there will no doubt be countless guys wanting to have sex with her. You can’t do anything to change this so provided she’s sane, trustworthy and committed to the relationship there’s no point in worrying about it.