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At the risk of sounding arrogant, let me just say that the best way to deal with this situation is to use a little horse sense. (Mark Twain preferred that to "common sense." "If it was common, everyone would have it," Twain opined.)
Ask yourself this, Concerned: if you kept them from boffing in the boy's bedroom, do you think that would stop them from having sex altogether? Or would it just drive it underground? And would that move give you more control over the situation, or less?
When this happened in our house, I told the teen involved to stop using the basement; aside from the candles burning next to the gas-fired furnace being something of a safety hazard, I told him that since the secret was out, his bedroom would be much more comfortable. Stay in the house. Use birth control. Don't let your girlfriend wander around the house naked. Otherwise, it was up to him.
The girl's parents? We told them. The girl's birth control? We supplied the boy with condoms; the girl's family put her on the pill.. Otherwise, we let them alone, under the principal that if they're gonna, they're gonna. Acknowledging it gives you some control; denying it gives you less.
Eventually, the boy moved on to other women, and the girl moved on to other men. Neither got STDs, and the girl didn't get pregnant.
I think you miffed a little, though, Gary, bringing up the legal issues first. Concerned is already wigged out; talking about discussing things with a family lawyer before addressing other issues is not going to wig him out less.
Horse sense, that's what I (arrogantly) say.