I can empathize with your predicament. Although my particular "asset" isn't nearly as visible as yours, I've had numerous images rejected from multiple online platforms. I've found that the images need to offer more of a full body view, and not so much of that one particular area. Trust me, you don't have to minimize it, it's still going to be noticed.
This is only my opinion, but contemporary women seem to find "gelled" hair, defined abs, good muscles, much more appealing than what's between your thighs...At least at first glance. You can showcase those yet still maintain some privacy if you try.
As to why these sites allow women in tiny bikinis that leave little to the imagination, or similar lingerie......It's simple, those sites are primarily run by, and for the benefit of, men.....With the intention of connecting those guys with women. They need to cater towards their target audience. Face it, except for a rare few places like here, there aren't many places that celebrate that portion of the male body.