I think most of you do not understand the concerns about the legal impact of legalizing gay marriage. It has nothing to do with the Bible - it has everything to do with the worry that such a legalization will weaken or destroy the construct of heterosexual marriage as it currently exists, with a particular focus on the male-female union for child-raising purposes (which is the main reason that marriage enjoys its special legal status).
The laws as they currently exist define marriage as the union between a man and a woman. The rationale that has been adopted for broadening that definition to include same-sex couples unfortunately cannot be stopped there. Lisa Simpson, the actual argument here is not that your relationship is being compared with a bestiality relationship – rather, it is that if one accepts the rationale offered to expand the definition of marriage to a homosexual relationship, that rationale cannot logically be stopped there, and would extend to relationships of bestiality and polyamory. There is a huge difference between the that and the comparison you implied, which I’m sure you comprehend.
The rationale to which I’ve referred above is the rationale that any consenting adults who decide they want to be in a legally recognized relationship should be allowed to do so. There is no logical or legal (believe me, the two are not always coincident) reason why this should be limited to two persons. More troubling still, there is no legal or logical rationale for not extending this argument to closely related adults (incest) or to an adult and a creature whose consent is not required for humans to act upon it (bestiality).
First, examine polyamorous relationships – especially as there is already an interest group lined up to argue for this extension the moment that any sort of gay marriage based upon the two-adults rationale is passed. Without the rationale of marriage between one man and one woman for child-rearing purposes, there is no premise on which to limit the number of consenting adults in a union to two. That number, without the rationale, is arbitrary and capricious.
Second, examine incestuous relations. While it might be argued from a utilitarian perspective that society has an interest in preventing siblings from procreating, it is problematic in that we do not stop people with genetic predispositions to birth defects from procreating. This rationale would also not hold for same-sex incest.
Thirdly, examine bestiality. While some will argue this will not hold because the animal is not a consenting adult, this does not matter. We do all sorts of things to animals that do not require their consent – such as kill and eat them. The only argument against bestiality is a moral one, and the consent principle would not necessarily serve to limit a “marriage” between a man and an animal (incidentally, this also applies to the invalidation of sodomy laws – the rationale is easily transferred to these other “moral” laws).
As you can see, it is the rationale behind marriage defined as between one man and one woman for child rearing that is the limiting factor. Any sort of rationale that tries to encompass “consenting adults” threatens to fall down a very slippery slope indeed – and one that only requires one step (not your common logical fallacy slippery slope argument of if this then this then this then this so watch out).
The above is just from memory from some articles I have read on the subject. I suggest you do the same to understand what you are arguing about. The main author I have read is Stanley Kurtz – just do a Google search with his name and same-sex marriage and you should turn up some interesting debates – Kurtz’s articles always link to the articles he argues against, usually by Andrew Sullivan, a gay conservative who argues for expanding marriage to include same-sex couples.