Also, polygamy (for both men and women) should be allowed, right now most countries ban it, while it's an outright discrimination of Muslims (an most likely some other religions).
Of course, it should be somehow limited in order to prevent exploits like, for instance, prevent some clever people from "marrying" multiple "spouses" to get various tax cuts they individually get shared through the whole group, thus reducing the taxing to some silly amount. Or, for example, a man "marrying" 4 (already married or not) women who have one child each, therefore making the "family" have 4 children and thus be eligible for a free bus ticket for all of them, among other things.
I don't think it should be officially called "marriage", but people should be able to validate the act in the church of their choice (whether the church agrees for that it the church business). There's also a matter of language, what we call "marriage" in English refers to a lot of different rituals which most likely have different names depending on where the religion originated from.