Now now, who are to say that all incest couples want children?
What about post-menopause mother who want to get married with her own son?
I may find them odd, but I would not judge them or stand in their way.
And what about bestiality?
I think it may actually be legal in many places. Disgusting, yes. But Illegal? Why? We do much worse to animals. We eat them. We test chemicals on them. So who am I to say some guy can't get friendly with his goat?
Another case to consider is poligamy. Many in the west find it disgusting and it is illegal in many places. But again, I say that for us to ban it we should have some rational basis to do so (and as Islam allows for poligamy, I think you'd agree with that). Some rational arguments have been put forth and should be examined. Personally I'd err on the side of freedom, and so long as women are not being forced into or abused in poligamous marriages (or men in polyandrous ones) I see no reason for me to interject or judge them.
Same with the muslim head veils. I can see rational arguments for and against them, but personally I'd go with allowing them so long as the women are not forced or pressured to wear them.
Just because I have no interest in any of the above (or in being homosexual), and just because I may be disgusted by each of them, doesn't mean I should judge them, and certainly doesn't mean I have any right to interfere with them.
Practices like circumcision on the other hand, and the other things lily listed, I may object and intercede. Why? Because an innocent non-consenting person or victim is involved.
So no, this isn't terribly arbitrary. It is really quite simple: I stand for freedom and equity to all people, and I don't need religion to get me there.