Emphasis on "the nasty."
After starting out in the equally depraved advertising industry, Saeki got a job illustrating for Heibon Punch, a softcore men's magazine. The magazine might explain where he got the idea of working with the medium of boobs. But his fascination with the macabre? Oh, he attributes that to spending his childhood with a perpetually sick father. It was either art or serial killing, frankly.
"Go back to sleep, boy. Nothing to see here."
When asked why he dedicates his life to giving people angst-boners instead of, say, literally anything else, Saeki responded that he likes to focus on "the vivid flowers that sometimes hide and sometimes grow within a shameless, immoral, and horrifying dream." To translate that from artist to English: There's probably a flower right outside your door, but a flower made of penises crawling into a geisha's eye-vagina? That was only in Saeki's head until he gifted it to the world.