In my country, Australia, civil unions are becoming less common as "de facto" couples have all of the same rights. Tax forms ask you if you're in a de facto relationship, child custody considers de facto parents/dependents, etc.
You got the causality wrong. Civil unions are becoming less common because gay marriage is now a thing in Australia. Most people don't give a shit about the religious aspect and if you go to a government register rather than a church service to get "married" there's virtually no religious implications making civil unions pointless. In Australia civil unions and marriage have been treated to the same rights for a long time, the whole gay marriage part was really the only difference.
My partner and I will probably never marry (or have a civil union) but we have no intention of ever breaking up. We just don't really like weddings/parties and see no need to even bother with a civil union.
I'm much the same, but you do need to consider your long term implications or potential life changes as well. The biggest difference in Australia between civil unions and defacto relationships is that the former is registered, the latter is justified, and that justification varies between states. In some states living together is enough, in other states you need to prove long term financial co-dependence.
I would still recommend getting a civil union, you don't know where your life can lead you. Such as my life which very suddenly opened up an interesting opportunity to relocate to Europe. And that kicked off a shitstorm of what to do with my partner. Defacto in Australia but now we're following the laws of another country. If we were in a civil union it would have been okay, however since we were just de facto we were limited into countries which we can move to. Additionally the bar for proving a de facto relationship to get a partner visa was quite high, along with quite ludicrous trips to the embassy because we needed to get proof that we weren't married or in a civil union in Australia (a letter of no-objection to marriage) to say we could be de facto somewhere and didn't have marriage interests elsewhere. The ludicrous part here is that every Australian state has its own register of relationships so we had to get TWELVE official documents issued to prove to the country we moved to that we could be considered for a defacto relationship.
It would have been easier to hand over one cert of civil union ;-)
Incidentally this is the same thing as people who insist being permanent residents means they don't need to get citizenship because the rights are 99.9% the same. Well it's that 0.1% that gets you in the news such as "X has been deported to a country he's never lived in" kind of situation.
Do yourself a favour, do not leave your life to the whims of government assumptions. Documentation is actually important in life.