Without sex there would be six billion or so fewer people on the planet, without death we'd be getting eaten by dinosaurs... and surviving. These are fundamental parts of the human condition, and they make us nervous. So nervous that over time we've built up huge complicated rituals about them like an oyster builds a pearl around grit.
Very like building a pearl around grit really, at least for me, because I find the rituals far more fascinating than the events they're ostensibly about. It fascinates me to discover that ancient Athenian courtesans were often named after fish, for example; or that tribes-people in Siberia once mummified human heads by covering them in gypsum. It's more than that though, I'm fascinated with the psychology of the rituals and the psychology of why we have rituals at all (nervousness is probably only part of it).
So, because these things fascinate me, I write about them, not necessarily graphically, not necessarily even obviously, but I do, because you know, writing about something I wasn't interested in would be weird. Also, because I like to write fantasy, I tend to make up funeral rites and societal attitudes to sex and sexuality to go with my imaginary societies, to me that's an obvious thing to do, they're universal aspects of the human experience and human or human-like society is probably going to have some kind of traditional view point, to a lot of other people it apparently makes me a freak.
Here's the thing then, I think I need to figure out a better way of explaining what I write about than saying 'I write about sex and death' because while in my head that's shorthand for 'I write about the human condition with specific reference to the special place in social construction of..." etc, etc in other people's heads it apparently sounds like 'I write really weird porn' which I don't... usually.