I had a deja-epiphany today. At one point I had had it sorted out, but then something just disappeared. Well, anyway.
I first understood the term "relativism" to be a simple statement of scientific fact: that morality is based on whomever chooses it (i.e. morality is relative to human construction). Ahah, but not so! To those among us who decry "relativism", it means two things: A) Morals are absolute (i.e. not created by man) & B) "Relativists" do not believe it is proper to force others to accept their morals. In this way they are able to conflate two very different positions and use one to bat the other.
What this generally takes the form of is, "Oh, so you think being gay is OK just because you feel like it? You probably think its OK to steal then too, right? Because it's anything goes!".
(Quick aside: Conflation is an interestingly common logical fallacy. One also sees it with objection to abortion as "murder". If abortion is murder, then people who have or support abortions are "murderers". But murder implies intent. Just because one person says squashing ants is murder, does not squashing ants a murder of me make.)
So, one can surely believe that morals are relative, and take a firm position on things. For all I know, we may one day find evidence of some moral absolute written down somewhere (although the problem will then be who wrote it, but let's leave it there for now). But as it stands there is no evidence for an particular moral. But we think and feel in pretty much the same ways. And that's basically persuasive enough for most people.
For instance, I seriously doubt there is any "moral" law about farting next to your wife. But she doesn't like it. So we don't do it. (OK, I often do but I accept that it is immoral.) There we go: moral defined by man (woman?). Is it proper for me to force my morality on others? Well, if you and your wife are "into" that sort of thing - then by all means, right? That's the beauty of creating your own morals. As long as you are a consenting adult then universal human rights begin to apply. But no need to get the UN involved in this, is there?