The slippery slope argument only makes sense if were talking about non-consensual sex. Why the raid on the Texas polygamists? Plural marriage by itself is technically against the law but it's not really enforced. The state may only recognize one "legal" wife but if you want to live with other women who are, for practical purposes, wives then your not likely to run into too much trouble. The problem is when you have closed communities where girl too young to provide legal consent are coerced into "marrying" men.

Personally I'm quite happy with just one wife. I wouldn't want more but if a man wants to have two or three wives or a woman wants to have multiple husbands I really don't care as long as everyone is old enough and informed enough to do so.

Closed polygamist communities often keep their members shielded from the outside world so even if they are of the age of consent they may not be informed about the wider world and be aware of their options. Still we have to draw the line somewhere so once they are 18 there's not a whole lot we can do regardless.

As for incest? How close is the relationship? I think almost every culture has taboos against brothers and sisters marrying (okay not the Egyptian Pharoahs but they were considered Gods so our rules don't apply) but first cousins, second cousins? I wouldn't do it but unless theres some recessive gene there's not a whole lot of reason to ban it other then the "yuck" factor.

Again it comes down to consent though. You can't marry your cousin or have sex with them legally before their 18.

"Man on Dog" sex? Well clearly this is disgusting and there is no way for a dog or barnyard animal to give legal consent so you certainly couldn't legally "marry" a dog, cat or goat.

Finally it comes back to gay marriage. Again this is a an issue of consent. If two people are above the age of consent and want to get married then they should be able to do. Period.

"Closed polygamist communities often keep their members shielded from the outside world so even if they are of the age of consent they may not be informed about the wider world and be aware of their options. Still we have to draw the line somewhere so once they are 18 there's not a whole lot we can do regardless."

A law legalising polygamy could include a requirement that new marriages require a reading of rights, with signatures from all participants, before the license is granted. To keep the polygamists from claiming unfair treatment, make it mandatory for all new couples (it isn't a bad idea, anyway). To account for common law marriages, people applying for benefits involving children or partners could be required to listen to a reading of rights as part of the process.