There is a long and dark history to moralizing and even legislating how and when consenting adults can have sex with each other.

So maybe it's a bit like discussing eugenics. It wasn't a particularly difficult topic when people like Plato were abstractly philosophizing about ideal societies, but it is a charged topic today because we have seen where that road leads in practice.

It's also easier to ask people to make shared sacrifices which everybody should bear equally.