IMHO, there's two sides to this coin: past and present. The following few paragraphs are from an Eastern European perspective, so it might vary with local history. I'll write it regardless however. :)

The history of marriage over here - is that of passing down ownership of resources. For feudal lords and merchants, marriage was about passing down power and wealth - it was strictly regulated for them, and often quite an oppressive institution (we can guess which gender it was more oppressive for, and which gender was considered a resource).

For peasants, it was a bit loose. The church had its laws but knew well - peasants didn't give a damn. They didn't have much wealth or power to pass down to children, and in some times and lands, the only inheritance was serfdom or slavery. So peasants, ironically, while being unfree people, had more freedom in their relationships...

...but they were not model people, and had their quirks. Often enough, marriage was entered when pregnancy had started. Peasants wanted to be sure that a couple was fertile, and did the formalities when that was sure enough. Not so late as to get the church pissed off, but not when the guys in fancy robes said it must be done.

Among anarchists, knowing this history, it is not uncommon to shy away from marriage, considering it a bit freaky, a bit like taking church and state to the bedroom, where there ought be two people.

Regardless, I know anarchists who have married. Especially when they were from different lands.