|Besides the blatant sperm-egg imagery, the kinkiness of Darth Vader's helmet, and the glowing condom/vibrator Luke Skywalker is wielding, I mean.|
Hence his view that no society should allow heterosexuals their rights and homosexuals theirs' and live and let live. Because such neutrality would encourage the idea that gays are equal to straights - which is material if not formal acquiescence to evil. That's how gay marriages hurt straight ones, in his view.The notion that gay marriage hurts straight marriage has always been laughable to anyone who doesn't buy into it, but I think I actually get where the theocrats are coming from. Think of the Star Wars trilogy. Awesome. Classic. Immaculate. (I know, I know, Jedi's not so hot, and the original is less a great movie like Empire than a special effects showcase, but just go with it. It's not like the history of marriage is so sterling either.) Now think of the prequel trilogy. Middling when not awful, a complete misfire that squanders the legacy of the original.
This last part is key. Many bad, or even potentially bad (think of the recently-announced Watchmen prequel books) spinoff projects have been rationalized by the standby that "even if it's bad, it doesn't harm the original work." But do we actually believe this? We certainly believe in brand reputation. We understand that a cultural entity is the sum of its constituent elements which can't be easily separated or compartmentalized away. Calvin & Hobbes is rightly remembered as a classic because Bill Watterson brought it to a close at its height. As a result our memories of it are still fond, and revisiting it is always worthwhile.
Contrast this (as I did off-handedly quite some time ago) with The Simpsons, which ought have cashed out at the end of the 90s after several brilliant seasons but instead continued on and is still with us today in a significantly diminished capacity. The diamond:shit ratio has suffered badly; Calvin & Hobbes was and is great, while no one can think of Star Wars or The Simpsons or The Matrix without wincing as if they just imagined two dudes on their honeymoon.
Looked at this way, it makes total sense that gay marriage/contraception/masturbation(?) opponents are so vehement that these behaviors not be given legal sanction, even in the form of neutral allowance. If you believe gay marriage is objectively wrong the way The Phantom Menace is objectively bad, you can't be live and let live about it. You "care" about the institution enough that you don't want its weird sequel damaging its good name, and so you'll fight like hell to see that Marriage 2: Electric Bugaloo doesn't come to pass.