FTR, finding a first world study on this is going to be tough, because it's completely and utterly regressive! It's like finding a first world study on feudalism or something. It's been abandoned because it's largely about one person having power and also spouse collecting. It's just so hard in terms of marriage to separate the issue from economics.
Now if a person just wants to have a bunch of sex partners, that's different. Economically and legally, at least.
If a chimpanzee could be scientifically proven to give informed consent to a marriage, I would support human-chimpanzee marriage.
The DNA isn't even compatible!
A lot of couples are incompatible, but we haven't banned civil unions yet. Furthermore, who cares? If Chimpy and Bob want to get married, then let them!
