Thanks again for all of the responses. Particular thanks to Taomist and polar bear for the links–I will check them out tonight.
To clarify a few points:
“Because I said so” is the standing rule of the house. But my wife and I have always been willing to discuss our rationale upon request and are willing to listen to counterarguments. If we’re persuaded to change the rule, the rule gets changed (this has happened in the past). If we’re not persuaded, then the rule stands, whether the kids agree with it or not. We do not believe we owe anyone an explanation for any rule; it’s just something we have chosen to offer.
My wife, my daughter, and I have dealt with my daughter’s breaking of the rule. She has lost some privileges as a result of that decision and has accepted those consequences without complaint. The issue on the table now is just the prohibition on sex.
As has been suggested by others, a rule may not be a good rule if you cannot explain the basis. My failure to articulate a reason for the “no sex” rule is precisely why I wrote the OP. That said, I’m not (yet) comfortable dropping the rule just because I can’t articulate a reason for it. I can’t articulate a reason why my 11-year old daughter can’t engage in similar activities that my 15-year old wants to, but I’m not willing to drop the rule there, so why drop it now? I fully acknowledge the logical difficulties of this position and my discomfort with it. It’s something we’re still working on–we’re not locked into any position. I’m not looking specifically for reasons to support my position that the rule is appropriate; I’m willing to consider that it may not be.
One thing I didn’t state earlier is that none of us–me, my wife, or my daughter–really know the girlfriend all that well. I don’t know whether she’s virginal and innocent, for example, as someone described her above (I have no reason to suspect she isn’t, though, other than the general unfamiliarity). She and my daughter met at summer camp and the weekend visit was the first time they’ve spent together physically since that week of camp. She’s 17–two years older than my daughter–which causes some concern on its own. Two years is ordinarily not a big deal, but my wife and I believe it’s a pretty wide gap at these particular ages. I don’t know to what extent the girlfriend is influencing my daughter, but as I’ve said, my daughter is usually a responsible, intelligent, and mature kid. Her decision to sneak into the basement bedroom really came as a shock to me and my wife, as we had discussed sex with my daughter in the past and she has maintained that she has no interest in it. Clearly that has now changed. Maybe it changed because of the girlfriend, maybe it would have changed anyway. My daughter says there’s no influence, but it may not even be conscious influence.
That would work, but it conflicts with my efforts to talk my wife into some experimentation now. 
Sorry if I missed any specific questions or comments. If so, I’ll get to them.