[tgb]

I think it comes down to a game theory style argument. Most people who would undergo this would do so unwillingly by pressure from family, religion, etc. and face large downsides. The small minority that actually desires to do this are hurt by making it illegal, but the large majority that would otherwise be coerced into it are spared.

Many laws banning sexual practices are more justifiable under such laws. If society were different, such a law would be net negative, but instead it's a good idea. Thomas Schelling's "The Strategy of Conflict" talks a lot about ideas like these style arguments and I would recommend reading it. For example, it is often illegal to take or display a picture of your voting ballot, since making that universally illegal makes coercing votes harder.

[fiatjaf]

That is a good argument for the existence of these laws, but anyway it is kinda strange that the LGBT movement promotes this kind of thing so enthusiastically. I don't imagine that people promoting these laws -- for example, the guy who made this website -- are aware of your argument.