Where do you see the "Nice Guy Syndrome" originating from? From a patriarchal paradigm? Is it little more than an urban legend? Have you had personal experience with the "Nice Guy Syndrome"?
I think your scenario differs considerably from the typical example of this type of thing.
Being in an open marriage, and having had many successful relationships or partners in the past, would probably make rejection by someone feel different than in other cases. The type of guy that would generally complain about the supposed nice guy problem, seems to be someone who is younger with limited or no sexual and relationship experience, so a rejection for them is a rejection of their only perceived option and a perceived threat to their being, rather than still having a marriage and many successful past partners and someone to go home to. They perceive themselves losing something in a category that they are lacking in, rather than losing something in a category that they're abundant in.
So, I don't know, but it seems similar to comparing someone with multiple current or past successful businesses and plenty of available capital that has a business idea go poorly and they shrug it off and move on, vs someone younger who went all-in on their only business plan and it didn't work out. Their places after that are very different, and so the impact on their perception will also be significantly different.
I've not really had to deal with nice guy syndrome personally, although I've witnessed it, where a guy complains that women are irrational, don't appreciate nice guys, etc. Maybe some of the people I've rejected felt that way about me without saying it, I don't know, but I've seen it directed at others.
It's probably a matter of displacement. Rather than simply accepting that not everyone will be attracted to him, or not asking himself if maybe certain qualities about himself are generally unattractive, it's a case of blaming the other person, feeling entitled to their attraction due to supposedly good behavior, and then criticizing the woman's attraction criteria if she isn't attracted to him. It's
her fault,
she's attracted to jerks,
she's irrational, basically.
I'm not sure what the main cause is. It's probably all different. It could be a personality issue, and I'm sure it's partially based on patriarchy and objectification. And in a lot of fiction, the protagonist is often a nice guy who inevitably gets the girl in the end, so in real life, if a guy views himself to be in that protagonist role and then the woman he likes doesn't reciprocate his feelings,he might feel cheated.
I don't view entitlement, whining, and objectification, as particularly nice traits, so it seems to me it's a matter of a person considering themselves nice when really, they often might not be. Then there are wacky ideas about how a man has to be a jerk to attract women.
It seems more of a prevent notion among men, with phrases like "friend-zoned" or "nice guys finish last" being well-known phrases. The female equivalent, though, would probably be when a woman is rejected by a guy who goes out with someone who is considered by the woman and other people to be more beautiful, and so the rejected woman and her friends might suggest amongst themselves that the guy is a jerk and only cares about superficial appearance. And along with that seems to be a notion that if one is very beautiful, they must have a balancing negative trait. Like, they can't be just as nice and beautiful, but rather, if they're beautiful, then they must be insecure, or superficial, or starving themselves to look that way, or whatever. So that's a type of displacement to avoid feeling hurt, where one considers the man who was attracted to a perceived beautiful woman to be a superficial jerk, and to consider that beautiful woman as probably having negative traits to counter-balance her physical attractiveness.
. She is perhaps the only person in this world that accepts me for who i am. That is a powerful thing.