Well since sex is such a integral part of pretty much everything I write, I could argue all the research I do is about sex. However, I recently wrote a 150 word vignette to turn my gf on one night. It worked but later I thought I could expand it into a piece of flash fiction erotica.
The vignettes MC was a female Island mage, with both her exact culture and precise form of magic use unspecified, the other two characters were a male and female from different cultures fighting over her. In the vignette the cultures of the two combatants were Aztec and Comanche. But for the expansion I wanted them to be warrior priests drawn from different and widely separated regions bordering the Pacific.
I divided the Pacific rim into three regions: Asia, North America, and South America, then began searching for cultures which I thought suitable. I eventually settled on a female Taoist monk from Five Dynasties and Ten Kingdoms China and a male Toltec warrior priest from Mesoamerica. While the female Island mage became a trans Polynesian sorceress. Then I had to work out a way to get all three together. I decided to see if an alt history where the Polynesians developed a mercantile trading civilisation linking the Americas with Asia would be a possibility.
This led to a lot of research into the development of marine and navigation technology along with the Austronesian expansion into the Pacific. After about a week or two I developed a rough timeline which could potentially make it work and allow me to set it in the Mid 10th century. Yes I had to bring forward the expansion into Polynesia by several hundred years, but that could be plausibly explained.
So after about three or four weeks research I was finally able to write the 500 word piece of flash fiction. Plus I had a pretty good blueprint for beginning the development of a brand new setting for my writing, one which I think has a great deal of exciting potential. Oddly virtually nothing from the original vignette survived the expansion, basically it was an entirely new piece.