Indeed. While I view the postmodern denial of universal human concepts as disgusting it would be extremely arrogant if humankind thought that it knew what is best for aliens. To quote Archer from Cogenitor: You thought you were doing the right thing. I might agree if this was Florida, or Singapore, but it's not, is it. We're in deep space and a person is dead.
As you pointed out arranged marriage might be forbidden on United Earth but it can hardly be forbidden in the Federation. It might be a stupid artifact from the past and not serve any direct purpose but Vulcan society is so orthodox and rigid for a good reason. Not our job to tell them to change their ways.
Furthermore humans are not even aware of all the various lifeforms that exist. They initially deny Horta, Data or holograms basic rights so how they be the judge of what is right and wrong for them.