Male - I think the first time was when I first tried my parents new whirlpool bathtub around 12 years of age. Apparently the jets were hitting just right. I had no idea it happened, but when I got out, I was the most relaxed I had ever been in my life.
On subsequent baths, I realized something was happening in my crotch region, and with some encyclopedia research, I figured it out. It became part of my regular routine for years.