If gay marriage threatens your marriage, it's probably because one of you is gay and your gay spouse is thinking, "What the hell am I doing hiding in a sham straight marriage when gay people are finding lasting love and legal recognition? Let me go find my true happiness and soul mate before I croak!"
If both of you are straight, it probably won't do a damn thing. Except increase the likelihood you will run into an gay person or couple at your PTA meeting or company barbecue. Which you always used to do by the way. You just didn't realize they were gay because they were hiding in a sham straight marriage or were the perpetual bachelor whose love life was a mystery in large part because they took care not to get too attached to anyone for too long, going instead for multiple partners. Something happens when you fall in love, stick to someone and place them above all others, in your life. That's when it gets really hard to be closeted. Living together, being seen together all the time and getting into situations where you have to speak for the other are going to raise certain questions. You will be outed! Ask a heterosexual, married couple to try and hide their relationship and spouse from the world, if they think it is so easy to do. Why do gay marriage opponents want LGBT people to hide from the world? Why their constant refrain of, "Just don't flaunt it in our faces."
Because VISIBILITY makes something become common-place and accepted over time. You know, like how you don't bat an eyelash anymore at an interracial couple. We forget it wasn't even that long ago when an interracial kiss between Captain Kirk and Uhura on prime time television caused a rain of hate mail to the studio from these "Godly" folk, claiming to be the protectors of tradition.
"The CHILDREN! Think of the CHILDREN, they were watching for goodness sake! How dare you show this filth on television!"
Viability of something that contradicts a tradition also challenges that tradition. It makes people start examining more closely what exactly are the sources of certain traditions and are they actually GOOD? The late 19th and 20th Centuries saw people starting to test and challenge the assertions of the "white", the "male", the "Judeo-Christian" powers that be. Then in the 21st Century it picked up momentum and another set of assertions by the "Arab", "male", "Islamic" powers that be, started to be challenged more rigorously as well. I know they prefer times when they were taken at their WORD about what they said is right and wrong. If they said miscegenation was wrong and the black race was inferior and backed it up with scripture they expected to be BELIEVED and their directives on that matter FOLLOWED. If they said the woman was created for the use of the man and is subservient to him and used scripture to back it up, they expected to be BELIEVED and their directives FOLLOWED. They long for their respective golden ages of supremacy, forgetting that it was not a golden age for any other demographic. They prefer times when the people they relegated to second class citizens STAYED IN THEIR PLACE! They prefer those days when the victim-less crimes they concocted to oppress people made law-breakers hide and sneak around in shame and fear of bigotry and violence.
Giving visibility to people who are rebelling against them and challenging their so-called "authority" and flouting how flimsy and false their legalities and assertions are, is like a slap in the face! It is like a salesman telling you there is no other model like his on the market and you see one drive past, proving him a liar. It is causing people to want to broaden their understanding, ask a lot of tough questions, expose contradictions and poke large holes in previously held beliefs! The powers that be have no way of dealing with their loss of dominion if this process of enlightenment were to happen. So, they are resorting to the time old method of tyranny and censorship. If something challenges their supremacy, they call it obscene and offensive or dangerous! It was exactly this kind of tyranny and censorship that gave them their power and authority. Their claim to power cannot survive an age of critical thought, widely available information and free expression. It cannot survive if gay people are living happy, healthy, honest lives, fully-integrated into society and actively making a public contribution or what they call, "Flaunting It!"