The variables in the Federation's case are universal prosperity and the clear ascendance of consensuality, not "open relationships." E.g. people who consent to be in a kind of relationship can have one, and those who don't, don't have to. So I don't understand what this is all about:
Nobody's being forced into one "attitude towards sexuality." Nobody's even being compelled to vacation on Risa and get freaky-deaky with people who dig casual sex if they don't want to. (The Risans are hinted at as being more generally sex-oriented than the Federation standard.) And presumably, though Trek was cowardly about showing this in keeping with Eighties American television norms, nobody's being forced into just heteronormative sex either. (Given the variety of species on offer it's in fact probable that there's something to suit tastes as catholic as those of Captain Jack Harkness.)If one thing has been illustrated to from years of counseling research attitudes towards sexuality are varied across cultures and individual experiences.
So, why are you "curious" as to whether a situation that allows variation without forcing anything on any particular culture is credibly described as "more advanced"? Do you have some other notion about what would be a "more advanced" situation? (I'm leaving aside "evolved," which is meaningless in this context.) If so, what is it?
Eh, it was more a question for clarification sake. The idea that the Federation as never "forced" their view of sexuality on anyone depends largely on the episode.
As for me, I'm largely libertarian, with a leaning towards preferring intact family units for the sake of raising children.
That said, I am always curious about sexual topics because I love learning about other people's points of view. Read in to that what you will (certainly something nefarious).