I'm a monogamous hetero cis man, and I've never had to advocate or filter for a polyamorous relationship. There is a pro-forma declaration of monogamy pretty early when I'm dating, but I feel like the contract has already been written and communicated in a million small and subtle ways before then, and from my limited experience, that the contract is non-negotiable.
I'm guessing there are subtle signals we put out that indicate where we stand on things, and maybe those signals mutually filter people out before a conversation even needs to be had, so maybe we're all working with self-selected population bias...
That said, the common complaint of straight cis women in dating is that there are a lot of fuckboys out there, who are just interested in sex, and surely the men quickly realize that being upfront about what they want is disqualifying on a near population-wide level, so they quickly learn not to communicate what they really want.
Hetero pessimism, in a nutshell.