I don't know in which Europe you live but "mixed-collar" isn't any more common here. Firstly the "mixed-collar" marriages in the article refer to educated vs uneducated individuals, lower classes especially in the US are much less likely to achieve college or higher education, in Europe with few exceptions (cough UK cough) it's not an issue.

But in Europe it's going to be just the same the chances of some one with say graduate or doctoral degree being in a long term relationship some one without a similar degree is fairly low.

People meet at school at in work, they usually have social interactions with their peers, and most importantly educated people prefer to date other educated people because they are more likely to share interests, world views, and could hold a conversation with each other.

And as far as true "collar" difference goes than you need only too look at old money families all those "new age aristocrats" while they might "sleep around with the help" they usually don't mix with others especially in countries where familial history is very important like the UK.