Since there's debate on equal marriage going on right now in the House of Lords, let me just give opponents of the idea a little bit of advice. If you want to avoid being labelled a homophobe - this supposedly utterly hyperbolic and unfair slander pressed upon you by all those nasty knee-jerk liberals that seem to be everywhere these days - then for the love of Gods, make sure you've thought through your counter arguments before you trot them into the public domain.
"What's to stop a man marrying his son?" is not a sensible argument. "Why not let two elderly sisters get married, too?" isn't a sensible argument. "Gay people can get married... to the opposite sex!" isn't a sensible argument. "Marriage is too important to risk, and so irrelevant that no-one should mind not getting to have it" is so baldly incoherent that it amazes me thousands of people have apparently signed up for it wholesale, though in their defence there's usually a couple of sentences in-between the two statements, most often about how gayness is the greatest threat to a child's upbringing since caramel-coated paedophiles.
Because here is the truth. If you spout this kind of idiocy - if you decide that your role in discussing an issue of direct relevance to hundreds of thousands of citizens of this country - then you are a functionally indistinguishable from a bigot. Because you've decided these people, who may or may not have had the chance to vote for you (recently at least) but whom you most certainly represent either way, are not worth the time it would take to consider their desires seriously. Because a historically repressed minority is asking you to give them something the majority has taken for granted for centuries, and you decided it wasn't worth more than five minutes thought on the drive into work to come up with a reason why you weren't going to bother saying "yes".
Show me the relevant difference between a man who wants to oppress a minority, and a man who cares so little about the oppression of minorities that they spend the minimum amount of time possible coming up with a slogan that wouldn't persuade caffeine-addled woodlice so that they can get on with their day. Show me the reasons I should give you marks because your position isn't "I hate you" but rather "I have no intention of thinking about you at all", particularly in this case, when your role in society involves precisely that.
When you tell a group of people that they're concerns are not worthy of the time you would need to understand them, you insult and belittle them. At the absolute best, you might be able to claim you ignore and smugly wave away the concerns of every constituency. Of course, if you do do that, then there are worse words for you than "homophobe", and everyone here knows exactly what they are.