So why not argue that instead of just shut Stieny out?
Yes, stienman opposes gay marriage on religious reasons. Last I checked, that was one of the freedoms you guys liked about your country so much.
He's also trying to have a rational debate about it. He believes God's One Message is more important than the rights of homosexuals (or not - I've heard Stienman state he's opposed to gay marriage, but not that he thinks it should be made illegal - I can be against allowing alcohol but that doesn't mean I must be in favour of prohibition). Most others here would say equal rights trump any other concerns about the matter. I agree with that second group, for that matter.
However, Stienman seems to want to put those concerns aside to have an open discussion about other reasons/benefits/etc about gay marriage. Purely economicall-logically, what are the benefits of gay marriage as recognised by law for a society? Mind you, you can have all the Vegas wedding chapel marriages in the world and they don't change anything (except getting some people's moral undies in a wad). A legal marriage has specific benefits and reasons. The state supports marriage as a means of stabilizing society, allowing children safe environments to grow up in, and so on. Other benefits of marriage are, for example, trying to keep people from ditching their SO when they get ill or grow old. It's also a way of supporting procreation.
I think a rational argument can be made that most of these benefits still apply when you look at gay marriage - and in fact, most of those reasons are stronger as counterarguments to legalizing divorce.
As Tegid said, I thinkthese days "suppressing procreation" may be in society's best interest more than supporting it - for more on this, go read Endless War, SF where due to overpopulation it's straight sex that ends up being looked at as a weird fetish, with all babies bottle-grown. It could happen, given a couple of hundred years.
As for the slippery slope, as stated by someone further up, it's not enough to call it out - you have to prove there's a stopping point somewhere on the slope. Consent is what most people seem to accept as the logical stopping point these days (and I personally agree), however, this most certainly wasn't always the case. It also opens a bit of a can of worms. Defining when one can consent isn't a simple matter. A 14 y/o and an 18 y/o having sex is legal in one country, illegal and rape in the next. Crossing a border does not suddenly make someone smarter or more responsible. Date rape is another example where the lines can get pretty blurred - one can't be expected to carry a breathalyzer to every party, so excluding both ends of the scale (stone cold sober and utterly wasted and incapable of standing up), how do you determine whether or not the other can still properly consent? Some animals, as has been pointed out, can try to engage in sexual activities with humans. Obviously this is about biological wires getting crossed (that dog does not really think your leg's a good partner), but good luck figuring that out. Some biologists can and do equate that to other instances of sexuality we have come to accept as normal, since our own sexual drives are "intended" to lead to procreation (note that I personally think the whole sex-for-procreation argument is complete BS, I'm just acknowledging its existence - I hope to still be having fun with my girlfriend when she's 75 and I'm pretty sure we won't be at risk of pregnancy by that time)
As for the original point - I actually have heard people complain about "Ladies and Gentlemen", as they felt they belonged to neither. I really do think there's a huge difference between being a bigot about something, being simply uninformed about it, or following societal customs which are at the moment perhaps not inclusive enough.
Closing addendum: despite it being bandied around on the internet a lot, bestiality is not legal in Belgium. There's no specific law about "having sex with animals", it's taken as part of the animal cruelty law and rape law, since animals can't consent.
