It's always on

The mating screening process of men and women is always on. In every interaction both literally rate each other subconsciously with a ranking bar.
Women are relationship creatures which basically means that they find god through relationships and children if this is not a too fluffy expression for you. As a result women tend to stay in relationships just for the sake of it with the next good guy around. God bless, because otherwise there would not be that much relationships. Men would be more likely to fuck them and then again follow their purpose.
Naturals know this. And that's why they treat women always like women whether her finger is right next to her or not. And I think it's totally ok to put out your natural masculine vibe as long as you are not hitting on her too hard. A healthy mindset at this point is: "I know you have a boyfriend, and I respect that, but I don't compromise to be a sexual human being and it is your choice what you do". It's her choice so you should not get too guilty for seducing her as long as you are straight forward with your intentions. She should be mature enough to choose her boyfriend if she does not want to ruin the relationship.
Unfortunately most of todays women have no honour. If their boyfriend is not around she will probably cheat. It reminds me of reacting more like an animal than proactively choosing one's reality.
So remember it's always on and you have nothing to lose. If it's the girlfriend of one of your best friends you get involved with and she even cheats on him with you, don't be afraid. If your best friend is somehow rational he should thank you for showing him what a miserable bitch she is.

PS: Oh yeah, one more thing. You could wonder why I do not bother making a move on anothers girlfriend. It seems not integrous. But you could be any other guy if it comes to cheating. And if you genuinely like her start a relationship with her and train her to become a better woman. If you just want to fuck her it's probably a violation of love and respect, but I'm not so sure about this one. I would probably do it anyway if I'm really interested. If you have any suggestions please leave a comment below.