Exaggerate it to the point where it's much more dubious.
Explain why the exaggerated case is fundamentally different from the non-exaggerated case.
Here, I'll give an example: People who are in love should be allowed to marry, regardless of gender.
How about "regardless of age" or "regardless of species" also? What if a 16-year-old and a 40-year-old really want to marry? How about if a man wants to marry his horse?
How about "regardless of relationship" - what if a brother and sister want to marry each other?
Age: The standard objection is "what if the older one in the relationship is just exploiting/manipulating/using the younger one?" Yes, I suppose that's possible - but what if it's 17 and 19? Many states still prohibit that, but it doesn't seem to be quite the same issue. Now how about 18 and 40 - legal everywhere, but should it be allowed? And what if it's a particularly mature 16-year-old? It seems to me that this law only exists because it's harder to prosecute "this person was exploiting this one" - but that's the intent.
Species: Well, we can't prove that the horse/dolphin/parrot/whatever wants it . . . well, maybe we can in the parrot's case. And we've been making progress on communicating with dolphins . . . is it simply that we don't want humans marrying animals? If so, why, and is it just?
Relationship: Whenever this comes up, people always yell about inbreeding. So here's a different situation: two brothers want to marry each other. No inbreeding here!
Now you come up with one. You're only allowed to argue my points if you can come up with your own controversial issue. (If you've got a really interesting one, post it on your LJ and gimme a link. :) )