Moral issue... hmmm....
Well, we know that after 25 years of trying, they haven't found any genetic evidence that people are "born this way." In fact, the strongest statement that I've heard from a geneticist, or neurologist, is people might be "inclined" towards their sexual activity, much like some people might be "inclined" to be a couch potatoe.
So, they cannot claim it's a moral issue because they are "born that way."
Worse for them, if this is simply a moral issue, then the discussion will turn to reclassifying a moral "wrong" to a moral "right". That will inevitably lead to comparisons with people / activities that we don't wish to reclassify (pedophiles?). I know from experience that such a comparison horrifys the gay/lesbian/transgender/Idunno/whatever camp.
I never understood the significance of the debate between whether it's meaningful that homosexual instinct is innate (some folks are born that way), or if it is a taste that is developed for any number (or combination) of reasons.
Nature or nurture as some folks pose it.
It doesn't really matter what it is. To say that it is a natural instinct doesn't mean that it is to be required accomodation by our laws. People have plenty of perfectly natural instincts that don't serve public policy - that they are 'natural' isn't a reason to accomodate them.
That they are developed for whatever reason also isn't a reason to accomodate them.
I never understood the fixation on both sides of the argument with the nature/nurture debate. It really doesn't matter what the reason is - there's no particular need to change the longstanding institution of marriage for 100% of the culture to accomodate 1-2% of the culture.
That's not to say we should institutionally be mean to that 1-2%. We shouldn't. But the legal fact remains: every homosexual has the same right to get married as any heterosexual: find a person of the opposite sex to agree to marry you, fill out some forms, pay some money, have a ceremony, and you're good to go.