Definitely elements of cultural difference here. But I also think it's a question of being respectful.

For example, there's nothign wrong with talking about sex. But I wouldn't talk about it in front of just anybody. I understand that some people will be less open than I am and adjust my behaviour accordingly.

Or to give a nudity specific example, about a year ago, I was at a wedding and there was a woman breastfeeding her baby. A man who she didn't know decided to walk over to her and strike up a conversation. Now, there's nothing wrong with striking up a conversation with a stranger. And nothing wrong with breastfeeding. But maybe that guy should have thought about whether that woman was comfortable talking to a strange man while her breast was partly exposed (spoiler: she wasn't).

I'm actually perfectly comfortable being naked. But I assume, just to be on the safe side, that strangers around me aren't comfortable seeing me naked. I don't rush to hide myself, but I don't stroll around with my dick swinging either. So many of these issues, especially when we're talling about male/female interaction, are just about a bit of consideration for how others might feel.