These are fem style male to female transformation style thong swimwear designs.
Mens swimwear Male chastity
The one in pink is me showing of my transformation to full vagina look. It is very flat and smooth and almost impossible to differentiate from a real girl.
The photo below is a model supplied by Koala and as you can see he is completely flat too.
I find these male to female transformation suits to be incredibly sexy and stimulating to wear. I have a full back bikini male transformation suit that I wear all the time to the beach here in Los Angeles and I always find it amazing how the attract attention and new friends like a magnet!