Yeah, the problem is (speaking from experience as well as theory), mononormativity is so strong a force that poly people often get pressured into monogamous relationships (consciously or not), and then only realise that they feel differently from mono people when already in a monogamous relationship. Usually there has been a declaration of monogamy, but assenting to that is not exactly always a free choice, especially since the cultural norm is that that is the only way a relationship can progress. So it's not pure implicit signals leading to self-selection, and explicit contract-making doesn't solve the problem.

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.

It does seem like this happens; it's also pretty bleak. It seems to lead to a lot of lying and deception, and destruction of the social fabric. Norms among queer and poly people seem to me to be more functional, but I am surely biased. See also !are_the_straights_ok@lemmy.blahaj.zone