I am the type of person who cannot separate sexual relationships from romantic relationships. I used to think the same way you about open/poly relationshios and also “friends with benefits”. Every time I’d seen one they ended badly with someone, like me, “catching feelings” and someone getting hurt.
However, I recently found out that a couple in my friend group has an open marriage. They attend parties together and very much appear to be a loving couple like any other and, I believe, they are. They just also date and have sex with other people sometimes.
I think it isn’t for everyone. I also think that often someone is dissatisfied in a relationship and they think they can make it work with a third or open relationship… and it doesn’t work out. But I’m convinced that for some people it absolutely works.