So it's all about you. This is a great "burden". I call it a confidence, and if you find it hard to deal with, tough. But it sounds like your niece has a more general problem than the porn (although if she's doing bareback, she might have a very big problem one of these days... was this a one-off or a regular thing?). The blackmail threats (that's what they are) need to be dealt with. This is unacceptable.
Prudie agrees that LW is nuts to be angry about the "burden", and gives some good advice for helping the girl, rather than worrying about how hard it is to bear this knowledge alone. Beurk.
Prudie says to get other parents to go with her to take the matter to the coach and to report her to her daughter's school (because this behavior is a warning sign for bad behavior with the perv's own child). I say: vitriol right in her eyes.
(That said, the post 1980s freak about guys wearing swimsuits is very strange... IMO it's all about internalized, or externalized, homophobia... girls can be barely clothed, but god forbid a guy should be seen having a dick.)
Well, this sounds like a high school thing. Are you in this dating thing to marry someone? If so, you just might (just) have some concern. But if you're just dating, what's the big deal? As Dan Savage says, almost no one ends up with their first partner, and every partner is the wrong one until you find the right one. So, duh, give the girl a break and enjoy this. And work to be a good instructor. And don't make her go pick up some scumbag at a bar just to get laid.
Prudie agrees, and cites Julia Child as her reference. Huh?
I say: write a letter and say all that. You've already written it. What's the worst that could happen? That should would reject you? That's no different from the current situation anyway.
Prudie says that you owe her an explanation for your silence, even if it's a stupid one. So just send this column.
I say you tell her you've got a crush too, on the curvy lifeguard at the pool. Make sure you get photos and put them up on your bedroom wall.
Prudie says to tell her to shut up a bit about her crush.