Department stores wanting to make money is universal; it's not hard to make the leap that someone could've come up with the same or similar notion in Amestria and at an earlier point in time.

This, yes; the wearing white thing, I'm still toying with. (Never mind that I've already written both the bride and groom wearing white at their wedding at least once... the whole story is under revision! Really!)

And if one wants to go with the whole "death and taxes" thing, taxes could be what keeps the state's interest in marriage as well as putting restrictions on the types of marriages allowed.

Hm. If I think of it as, what is the Amestrian social attitude, or the aims of the Amestrian government...

The Amestrian government is restrictive; it does not seek to make people happy. In fact, in some cases, it actively seeks to make people unhappy, to promote the kind of desperation that would lead people to create the Philosopher's Stone. While I imagine they probably don't take the Crowley approach to making people miserable (traffic! cell phones! annoying internet memes!), it's certainly not in their interest to make marriage all free and happy.

And if their goals include population growth so they have lots of wee soldiers to throw into war, that would be a great excuse for m/f marriage only.