I was thinking in terms of criminal migrants from where ever exploiting it to bring in their friends/family. I think on a 'traditional' level a man would generally have been the one marrying a woman from another country. The man would also generally probably want to remain in a relationship and although in some cases might marry to just get someone in and help them obtain legalization, I would say more times than not that is a rare situation. It is just one more loop hole that yes although exploited before this same sex marriage, could really be abused. For example, I am ISIS and obtain legal papers and become a citizen, marry another fighter (which under these circumstances they would be allowed to do although Homosexuality is forbidden) and just one by one obtain legal citizenship through this legal system.
That's exactly what the guy in the tween was saying, his brother or cousin or someone was an illegal who became legalized, and married one of his friends to get him into the US.