Yes, no rational person could possibly have anything against a relationship they have nothing to do with. Sure, there are plenty of irrational people who do, but it's impossibly to have a productive discussion with irrational people, so we'll not make provisions for them within our conversation. However, the question is not whether you mind if other people have open relationships. The question is why you would attempt to impose a closed relationship upon your partner you claim to love if he didn't want it.

As I've said, you are certainly allowed to make that decision for yourself. But why are you allowed to make that decision for your partner?

Yes, I would be honest about the expected terms of a relationship before agreeing to be in one. Only a fool would accept any agreement without being fully aware of its terms, and that goes for a relationship just as much as a business contract. That doesn't mean I'm going to bring it up on the first date, or second, or fifth. It doesn't mean I'm going to bring it up before having sex with someone for the first time, either. But when the topic of a committed relationship comes up, I definitely make it clear that sexual exclusivity is not part of that arrangement. Sexual exclusivity is not commitment. It's just sexual exclusivity. Commitment is something else entirely.