What's the connection some individuals make between same-sex relationships and incestuous relationships?

Fmr. Sen. Rick Santorum, possible 2012 GOP Candidate Fmr. Gov. Mike Huckabee, Republicans for Family Values, and some likeminded individuals (even on fluther) equate same-sex relationships – or the legalization of same-sex marriage, to be in the same category as incestuous relationships.

In researching this question before asking, I found this article: Is homosexuality ethical? If so, what differentiates it from incest? More specifically an infertile incestual relationship that has two consenting adults.

As a queer person in a same-sex relationship, and a survivor of childhood sexual abuse, I cannot fathom the connection between homosexuality and consensual incestuous relationships. To me, they are entirely two different things.

So, how do some opponents of homosexuality equate same-sex relationships to incest? What are the logical, moral, and ethical grounds for such a comparison?