I have heard the comparison many times, usually in the form of a slippery slope argument. My headline is the main argument.
In terms of slippery slope, if it were regarding polygamy I would agree with the possibility. If marriage isn't specifically about kids, maintaining lineage, combining families by blood, etc, then the definition becomes fluid. The first step is gay marriage. But then, marriage has changed. It is about love, life, sharing, support, etc. Not the more base survival imperatives.
I think the bestiality argument is more for shock value than anything. They are worried about the dilution of marriage, when they should be more worried about vegas weddings, internet ministers, escalating divorce rates, single parents, then they are about two loving people who happen to be the same sex taking their vows more seriously than most do these days.
They are both completely different things. One is about two people of the same sex falling in love and wanting a relationship and the other is about getting weird with animals! I'm not even sure how you'd even compare them. Homosexuality is genetic and the other is just plain gross and nasty!
Homosexuality is a perfectly natural desire and expression of love between a two humans of the same sex. Same gender sex is in no way related to interspecies sex. There is ample scientific evidence that suggest homosexuality is caused by processes of brain development. There are many different species of animals that exhibit homosexual behavior, not just humans.
There is no comparison to be had between bestiality and homosexuality. For a human to have relations with an animal is them having sex with the animal with no consent. This is rape. Two consenting people of the same sex is an act of love. These two things are in no way similar.
Two people who are consenting are not even remotely comparable to a person and an unable to consent animal. I don't care how you feel personally about LGBTQ people -- there is no reason to ever, under any circumstances, compare a consenting relationship to one that is essentially animal rape.
I do not believe it is acceptable to make comparisons between homosexuality and bestiality. Homosexuality still deals with sex between two humans, while bestiality refers to a human having sex with animals. The two are not comparable in any real way and they should not be compared. I certainly wouldn't find a persons statements credible if they did compare the two.