I think it’s the same root cause in both cases: sex and religion are things people invest a huge amount of their personal identity into, spend a lot of time, effort and money on, and often agonize over.

Somebody coming along and saying “I don’t believe in that/I’m not interested in that” feels like an attack regardless of how politely it’s worded. What the other person hears is “one of the fundamental tenets of your identity is fake and you’ve wasted your life caring about it so much, you fucking idiot.”

The happier and more confident the person seems in their position, the more threatening their adoption of that position seems, which is why a lot of religious people have this weird insistence on believing that atheists are secretly miserable without god. (Do ace people get this too? I imagine they might).

I think this is why the mere existence of atheism or asexuality (or to a lesser extent vegetarians, vegans, polyamorous people, people who don’t like the country you live in, sports factionalism, people who don’t care about cars or video games or books or money or alcohol…etc) is often met with such a knee-jerk hostility. To the insecure person, just stating that opinion feels like a literal attack on a core component of their personality.

(And actually now that I think about it this explains a lot of transphobia, because a lot of people put a lot of stock in living up to gender roles and expectations, and the existence of trans people and nonbinary people is taken as an implicit statement that all that effort was wasted on something that isn’t actually important after all)