Guy has strong convictions. Good for him. She will most likely cheat on him.
Open relationships rarely ever work.
They NEVER work from what ive seen. (Couple chick friends who tried, both of which I was banging whenever they got upset at their BF)
A guy will come along whos better in the sack and shell inevitably start getting more attached to him (and claim its emotional without realizing shes just addicted to his cock and her emotions are just following along) and when that happens, shell slowly start to emotionally distance herself from her BF until she doesn't give 2 fucks about him, then shell leave him for other dude.
Then when other dude gets bored of boning her, shell breakdown and come crying back, and hopefully the dude refuses to take the slut back and lets her wallow in her fucked up situation she created. But most guys dont, because the guys that let their GF bang other dudes are usually pussies anyway.
Other thing that happens is the chicks go psycho when seeing guy hooking up with others.