When I was in college, I dated a guy who never really said he wanted a non-monogamous relationship, but seemed fixated on the open relationships and polyamorous lifestyles his friends were in, and it really got in my head that that’s what I wanted, so one day I just broke down crying and said I didn’t want an open relationship, so he said he was fine with being monogamous. We were only together for a couple months though, because it really seemed like he still wanted to garden with other women, or was curious about what it would be like to garden with other women, and honestly, while he was a nice guy, he wasn’t the guy for me and a part of me really wishes I’d just walked away when it became apparent we wanted different things, and calmly told him “you seem interested in non-monogamy, but to be honest, that’s not for me, sorry.”
I do wish people like this were upfront about it from the beginning. One could argue that it’s problematic to have monogamy be the assumed default, but the fact is, that’s how it is right now, so if you don’t want monogamous people catching feelings for you and then having to break their hearts when you finally tell them you’ll always need to garden with other people to be happy, yeah, you need to disclose this preference for non-monogamy on dating apps, or at the very least, on the first date.
In fact, I’d be surprised if that’s not part the ethics. I haven’t read The Ethical Sl*t but I kinda figured one of the general rules is that you need to tell people up front that you prefer non-monogamy.
But to answer your question about what would have happened if you hadn’t called his bluff, eventually you would have gotten to the point where you want to DTR, and at that point he would have told you “oh, I’m actually not into monogamy” and explained that being in a committed relationship with him means letting him garden with other women, or being okay with him having serious relationships with other women. It’s generally assumed that until you have that talk with someone else, it’s fine to casually date other people, as long as you’re being discrete about it.