Why does talking about sex make some people feel uncomfortable?

Short answer: Consent.

Slightly longer answer: religious prohibitions, potential slut-shaming and/or sex-negativity, triggering of past sexual abuse and trauma, etc., and so on.

You never initially know what will or won’t make someone uncomfortable. Because you can’t, and since sex is a highly personal topic, it’s always best to ask first. Say something like “Is it okay if I talk to you about ‘X sexual topic’?” and then abide by other people’s comfort level, reactions, and answers. Don’t pressure people into discussing or revealing things they’d rather not talk about. However, if you get a “yes,” then don’t let social conventions stop you from having your conversation. Just make sure you have enough privacy to avoid unintentionally involving bystanders who’d rather not hear what you have to say about sex.

Best of luck.