1) If you'd accidentally sucked off your Dad you wouldn't tell a soul. "well you'd probably tell your best friend and they'd tell somebody else..."
2) Gloryholes are mostly found in cruise bars/sex clubs. Why would you be in a sex club with your Dad? "they were on stag and hen dos and probably didn't realise they'd gone to the same place ..."
3) The whole point of a glory hole is anonymity, why would you build one with a feature to remove that? "because it's a sex club so some people might want to watch ..."
4) Switchable glass is quite pricy, you wouldn't put it in a place where pissed brits on stag/hens are likely to break it. "it's probably toughened glass, that's not dear (expensive) ..."
Honestly, it's an awful thing to say but it's now impossible to have an intelligent discussion about anything sensible in our office. The one who gave most of the answers above asked me on Monday why, if you put a Goldbear (jelly sweet) in saltwater, does it swell up overnight. So I said it'll probably be osmosis and she asked whether that was a real word or had I made it up? I explained it was what makes your fingers go wrinkly if you stay in the bath too long and told her how the process worked. She still thought I was winding her up so I had to Google it for her before she believed me.