Same-sex marriage and incest *are* comparable

One occasionally hears opponents of same-sex marriage portray same-sex marriage as a sort of thin-end-of-the-wedge; "If we allow same-sex marriage, what next? Would we let a brother and sister get married." All the right-on lefties then fall over each other in their haste to point out how stupid this is, same-sex relationships and incest are completely different things. But are they? Can we really think of any rational (and I stress the word 'rational') reasons that incest is, in some sense, wrong? Is there any justification in condemning what two consenting adults do, even of they are related by blood? Possibly the most obvious such argument is that genetic deformities are more likely in the children of incest, but this clearly is an argument that having children in an incestuous relationship is wrong, and it doesn't hold as an argument in principle that incest is wrong; a couple could simply use a condom, and the argument evidently doesn't apply to a gay couple, or in the case one of them is infertile.
In fact, I don't think any rational argument that incest is wrong exists, any more than a rational argument that being gay is wrong. So I would say that same-sex relationships and incest are comparable, in that they are both disapproved of by society for no good reason.
And if you're not convinced, and feel that incest is 'just wrong', then take a moment to reflect that that's precisely the sort of thing people have said in the past about interracial relationships, or being gay.  

My relationship with my cats

I have three cats, and I have a very different relationship with all of them. First of all there's Darwin. He is not, I hasten to add, named after Charles Darwin. I first got Darwin when I was three, long before I'd heard of evolution; he is in fact named after a raccoon in a children's book called Detective Arthur. Me and Darwin are really quite close, he'll always sit on my lap, and nibble my finger, and be generally very affectionate.

Now, this is possibly the polar opposite of my relationship with Sooty. He and I rarely do more than pass the time of day, if that. The one frequent sort of contact we have is when he wants to go out. Usually he'll wander up to the door and say bruskly "Open the door, would you." To which I'll reply with a jovial "Right-ho" and he'll dash off without so much as a single word of thanks.

Mr. Tumble is quite a bit more friendly than Sooty, but he is rather indecisive. A typical conversation will go something like this:
[Mr. Tumble wanders over to the door]
Mr. Tumble: I say, would you mind awfully opening the door?
Self: But of course!
[I open the door, and Mr Tumble stands looking out]
Mr. Tumble: Actually, I think I'll just have a little morsel.
Self: Please yourself.
[Mr Tumble wanders over to the bowl, and I close the door]
Mr. Tumble: Look here, there's no food in the bowl.
Self: There is. Why, the bowl is more than half full.
Mr. Tumble: Well, so it is. But it doesn't matter, I think I'll just take a breath of fresh air

The conversation usually ends with Mr. Tumble being forcibly ejected from the house by a rather exasperated me.

Maths, physics and chemistry

The great physicist Richard Feynman once said “Physics is to mathematics like sex is to masturbation.” This is a view I strongly contest. On the contrary, Maths is like sex with a woman, Physics is like sex with a man.

Mathematics can be beautiful. It's beauty can be a high elegance; regal, almost. Or it might be what we could describe as 'pretty', something that makes us smile whenever we see it (which is by no means an inferior sort of beauty). But either way, I think maths has a distinctly feminine aspect.

Physics on the other hand, whilst certainly not inferior to maths, is very different. It's less perfect, more rough and ready. More likely to bend you over a table. Distinctly masculine.

But if maths and physics are both like sex, albeit different types of sex, then chemistry, with its imprecision and lack of deeper truth, must surely be like masturbation. Masturbation whilst watching particularly low quality pornography.