In the end most countries/places that have civil unions do seem to move onwards to marriage sooner or later (of course, the difference between the two is not the same everywhere; in the countries of the Code Napoléon where you have an obligatory civil marriage prior to your religious marriage ceremony anyway, the situation is quite different from the US where your religious marriage ceremony is the same as the legal one). But it definitely removes some of the sense of urgency, the Danes didn't need to be at the forefront for gay marriage anymore as they had their civil unions.
I can't remember the exact quote or who said it, but I'm thinking it was Churchill - that a young man who isn't left-wing has no heart, and an older man who isn't right-wing has no brain (or what did you expect from a politician who started out with the Liberals and then became a Tory icon). That may hold even more true for Europe than the US, I think our youth is probably even more uniformly left-wing than yours, due to the lack of popular right-wing ideology of a kind comparable to Republican ideology. But you are right that at the end of the day it's less about going from left to right and more about going from idealism to pragmatism, regardless of which side you start out from.
The gay marriage thing, though, is not even really about ideology. It's about young people having grown up with the idea that homosexuality is something people are born with, just a harmless "deviation" from the norm like being left-handed, and hence not having a problem with it.
I think what really bothers me about the gay rights cause and how people, myself very much included, think about it, is that it's so popular because you can be so delightfully black and white about it. When a country legalizes gay marriage, us 21st century youngsters who don't see homosexuality as morally bad in any way can cheer at it wholeheartedly and see it as a victory over reactionary opponents, which doesn't seem to have any negative side effects or real political price. In an age where politics in general is so complicated and always involves so many trade-offs and small victories here compensated by concessions there, it's just nice to have a cause where you can fully enjoy the whole us vs. them vibe and get the thrill of real victory whenever a place passes the legalization. I think I could give you a fairly accurate list of which countries in the world have legalized gay marriage off the top of my head, and even mostly in the correct order - which when you think about it is absurd. But I really don't think I'm the only one.
Okay, it's clearly very late, I'm just rambling at this point... time for bed.