it’s a deeply pernicious cultural framework that leads one to only value a romantic partner insofar as they might eventually become one’s future spouse.
Another issue that causes people to be dismissive towards open relationships is the idea that if your partner is having sex with someone else, it should be some kind of affront to you. Personally, I don't see why this is the case, especially if you're temporarily geographically separated from each other. Certainly, if you made some kind of promise to have an exclusive relationship, there's the fact that the other person is taking their promises to you lightly. But in the absence of such promises, nothing bad is happening. (A corollary of this is that it can be unwise to make promises of fidelity to give your partner security against such an affront.)
On a more personal note, I'm quite comfortable with the idea of my girlfriend having sex with lots of other guys, especially if I'm unavailable for some reason. (If the guys were jerks, there might be a problem, but assuming that's not the case, we're fine.) Girls are cool, and sex is cool. So girls who have a lot of sex are awesome, and I could happily be in a stable relationship with one of them.