Point of order: a "boyfriend" is nothing. Truthfully, speaking, a boyfriend is a sham concept grown out of the abominations of Rome. Boyfriends do not own their girlfriends, and girlfriends are not taken. That is an illusion, just as monogamy-only is. The shriekers (not you, Samuel) should therefore calm down.Also, look at this from the woman's perspective:
You are proposing encouraging a woman who is already taken (has a boyfriend) to flirt with other men. You are in other words proposing teaching a woman to cheat, or at least desensitising her to the idea.
However, perception is reality, and so long as we assume she has the perception that she is "taken", your point is valid that she is practicing now to actually practice adultery later if she openly entertains additional relationships while "in a relationship". That said, I also think an honest look will reveal that most people's perception of the unwritten rules of dating is fluid at best, and unable to coalesce into a clear standard because at its foundation is a shifting pile of lies.
So, this discussion is in a gray area and probably can't find common ground.