Well, multi-person marriages are common enough in many countries. In the case of the UK, when people in such marriages come here, the marriages aren't recognised. Or rather only the first one would be. (It happens quite often, especially in relation to Pakistan.)

This just brings up the truth that "marriage" can mean a lot of rather different things in different parts of the world. Just because it's called a marriage in one place, and is legally recognised, that's no guarantee at all that that'll be true elsewhere.