The thong-strap could be massaging the prostate, but more likely the seminal glands. The seminal glands provide the thin, "pre-cum" fluid. Just try loosening up the thong-strap by not pulling it up so much in back.
The other thing you may need to do is get something that would keep the fluid from seeping though your dancebelt. The fabric stores have that vinyl that is backed with fuzzy cotton. It's used to protect tabletops, but is quite flexible. Try a layer of that between yourself and the inside of your dancebelt.
Then there's the old joke among urologists: Old Sal comes in to the office to complain. "I leak, and when I wear white pants it shows." The urologist replies: "So don't wear white pants!"
The truth is that moisture, whether it's sweat or whatever, show more when you're wearing light colors. I (almost) always wear grey in class, which gets darker with sweat as class progresses. But my problem, the sweat doesn't work its way through the thick dancebelt, so my crotch stays light grey.
The solutions can be simple.
balletboy
PS: I don't think the "gay" has anything to do with it. When in class I rarely think about the fact that I'm gay. And I harumph when the teacher is showing waltz steps and tells the class "Imagine youself waltzing in the arms of your man." We don't waltz together. When we dance together it's much more like dirty dancing.
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