Because if I'm honest with myself, I genuinely enjoy sexual things. Sexuality is not something you can just switch off. It's a part of you from the start, and sooner or later you discover it. My stories have recurring cliches and fetishes. This is not really by choice. Artistically it's pretty dull.
The true reasons are fairly psychological and personal, and far more inter-related than you'd expect. It might surprise some people, but I actually have an immense hang-up when it comes to rape in real-life. Specifically involving grown men with females. It comes from all the feminism I attempted to study as a young idealistic teen. In my naivety, I generalized and aspired to be a person that girls would like; Because I liked them, and I wanted them to like me back. The problem was that "feminism" (especially just after the 1980's) actually had many different interpretations. But at that age I didn't know that. I thought it was all one thing. Feminism presented itself that way. I only knew that it was something good and that I should care about it. The most vocal perspectives were pretty extreme, but I never ended up hating myself... not quite. I knew for certain that my intentions were good. But it all left me with a relentless nagging doubt that no woman would ever believe that my intentions were good nor could they relate to the things I felt, that they would never truly be comfortable around me. So I completely gave up that hope. It pretty much destroyed by sexual self-confidence. I chose to use art as a deliberately harmless outlet for my sexuality from that point on. An alternative to something I was led to believe was inherently dangerous and unethical in real-life... at least when coming from a guy. The flaws of these old notions are mostly obvious to me now after all these years. But old habits die hard, and your first impessions form the foundation that you compare everything else to, and have a pesky way of coloring all of your thoughts from that point on, even when you are aware of them.
Sexually, I have no goal. I don't care about changing the world with my art or teaching people a bunch of profound insights. Life does that on its own. This is just simple selfish play and honest happiness. It doesn't need to be anything else. If other people happen to enjoy my stuff, then great! If not, there's no shortage of other great artists out there. We live in wonderful times with a luxury of choices.
Beyond that, I have already accomplished almost everything I ever dreamed of as a kid. I can create platformers like Super Metroid. I can create RPG's like Chrono Trigger. And it's all easy to do. Sure, that's all pretty low-tech by today's standards. But happiness isn't about chasing all the things you don't have, it's about recognizing all the things you do have. When I was a kid the most impossibly amazing thing I could imagine was a game console that would play both SNES and Sega Genesis games... and we're already light-years beyond that! ...no complaints here.