The example that they gave was gay sex and, even more specifically gay marriage. They were concerned because they felt libertariansism left no room for rebuttal against gay marriage and felt that, if "normalized", it leads to this: Paedophilia: bringing dark desires to light
Other than both being "sexual", I see no connection, but, OK.
He asks "If it happens in private between consenting parties, what argument can full-throated libertarians make against the act? Would they even offer an argument?"
Some offer arguments, but I can only speak for myself.
I read the article and found it very interesting... and then it hit me- the basic difference between me and your friend. He was offended by the information and opinions presented and I was not. I took in the information presented and gave it consideration. He, apparently, couldn't do that, but only saw it as confirmation of his worst fears.
The main case against forbidding gay marriage should be an easy "conservative" fit: how on earth can the claim be made that government should be in the business of licensing private and very personal vows? It isn't that gay marriage should be "allowed", it is that government should never have been allowed to regulate, sanction, or ration marriages in the first place. It's like licensing churches. Or guns. Or cars and travel.
Beyond that, if something is truly consensual (and it needs to be informed consent, without deception) and doesn't harm either of the participants, how can you seriously advocate killing people over it? And that is what any "law" against something leads to. All "laws" are enforced by death. Sure, you can usually avoid that outcome by complying early in the enforcement process, but eventually, if resistance is made, armed government employees will arrive to either "arrest" you or kill you in the attempt. Don't believe me? Find a minor "law" and openly defy it around the government employees who are "responsible" [sic] for enforcing it- and keep refusing to comply each time they escalate their enforcement attempts. Do you think they will shrug their shoulders and say "Oh well..." as they walk away?
His complaint that libertarianism leaves "little room for rebuttal" against gay marriage, because "What happens in the bedroom between consenting adults is of no concern to the true libertarian" seems misguided. Why would anyone want to rebut that? Because it offends them? No one has a right to not be offended. I don't worry that someone might be watching MSNBC or FOX news in the privacy of their own home, even though the thought offends me and I think it can cause wide-ranging problems out in society. Until someone is actually harmed, it just isn't any of my business.
Which gets back to the article he linked to.
If it were shown beyond a reasonable doubt that pedophilia, when engaged in willingly with INFORMED consent, did not harm either participant, neither physically nor psychologically, why use the lethal force of government (which is known, beyond a shadow of a doubt to cause harm to almost everyone) to prevent or stop it? Because it offends "the majority"? Gun ownership seems to offend either "the majority" or a very vocal minority. In itself, gun ownership harms no one. Until the gun is used to coerce or attack an innocent person- one who does not deserve to be harmed right now because they are not attacking or stealing. As long as no one is being harmed and everyone is engaging in consensual acts, you can be offended all you want, but you have no right to use force against them. Nor to delegate someone else to use force that you don't have the authority to delegate. You can't delegate something you don't possess.
I suspect that your conservative friend, if he were honest, would admit that he will never believe any studies that show "no harm from pedophilia". He will continue to "know" it causes harm, no matter what. And, in that, he may be right. I simply don't know.
So, no I wouldn't make an argument against pedophilia, in and of itself- although in the past I would have. No child "belongs" to me, just as I didn't "belong" to anyone when I was a child. Would I try to convince children that I thought it was a bad idea for them to engage in sex? Yes. Would I try to talk a pedophile out of acting on his desires? Absolutely. Just because the thought bothers me personally. Do I think it is somehow worse for a child to engage in sexual activity with an adult than with another child? Not really. And I also recognize that all children engage in some level of sexual experimentation or play. I did. It was completely consensual, it didn't harm me, and I don't regret it. Where would you draw the line between "normal" sex play and "perversion"? Where do you get the authority to draw that line?
And, even if it does cause harm, if there is informed consent with both parties, I wouldn't send The State after them. I would step in, myself. And I would accept any consequences that came from that.
Tattoos and piercings cause visible harm. Damage to the body that you can see with your own eyes. It offends my aesthetic sensibilities to see them, especially when there are "too many" of either (or both). I think that people who get multiples of either don't understand what they are doing to themselves. They invariably disagree with me on that point. I would still do my best to argue a person out of going overboard with them. But I know where the line is. I respect that line even when doing so makes me unhappy. Which is increasingly rare.
This is a taboo subject, and one I address with apprehension. More than any other subject, talking about sex leads to hatred and accusations and drives people away. It is the most emotionally charged subject I have ever found. Which means, I suppose, that it is important to talk about.