Since they weren't studying the male voles in this case, it didn't really matter if the guys were previously bonded to other females. Already bonded males will often still have sex with a passing female who is not his true mate. But he won't bond to her, because his brain was changed by his first bonding process. (and not just the oxytocin receptors either, but that's a topic for another post)

Yes it is not just the sex that changes the brain, that's true.

But the change is in oxytocin receptor expression in the neurons. It's more than just "there is a lot of oxytocin in the brain" it is that the brain is in a state where it can "sense" oxytocin better.

Actually there is a lot of research on the neurochemistry of male prairie voles during the bonding process. Just not in this study.

1. Interesting question. I don't think it is known yet whether other (non-pairing) animals have this same brain change due to sex. Actually it is not clear from this study how long this change lasts. It might not be permanent.
2. Also not known, but it would be interesting to see how variable the brain changes and the 'strength' of the pair bond is between voles. Do some voles pair more easily than others, are there specific pairings that work out best (Vole soulmates?)
3. Cuddling can definitely increase oxytocin levels (mothers breastfeeding is an example), so it wouldn't be too surprising if cuddly animal families (bunnies or grooming monkeys, for example) had different oxytocin receptor patterns in their brains. I've even wondered if colder climates facilitate pair bonding because you are more likely to be cuddled up with someone for an extended period of time. (Go to Alaska for a honeymoon, not Aruba). I have absolutely no data for this, it's just a possibility that has crossed my mind.

Haha. It really is more of a 'second date' sort of preference here.
One thing that would like to know is does 'bonding' to one male vole prevent or weaken the female's ability to bond to another? For the male voles bonding to the females it seems to work that way. A brain change after bonding/sex seems to reduce the male's chance of bonding to another female even if her has sex with her. Of course that is a topic for another whole article.

Great question. Sorry for taking so long to answer comments, I was in a dark room shooting lasers at neurons.
I don't think anyone has tested to see the minimum amount of time needed for a bond to form. In this article it was either 6 hours + sex or 24 hours without.