I can give ample reasons as to why neither should be allowed.
This is exactly the reason, or at least one of the reasons, why I have come down against classifying same sex relations as indeed ”marriage”. The risk of the slippery slope demands that we review the subject of same sex marriage in the context of: exactly where it will lead.
We can safely assume once Gay ”marriage” becomes a recognized norm, every other special interest group will demand a recognition of their lifestyle as well. What is to stop Muslims from demanding one of the key tenets of Islamic sharia, polygamy? Demanding that it’s their civil right to marry multiple times, any politician who voted for same sex marriage rights will be hard pressed to defend an anti-polygamy position.
More here. H/T: Fjordman