In theory, this is a hot idea and, if I ever actually traveled with a companion, I'd consider it. Sex at 30,000 feet? Why should your ears be the only thing to "pop?"
The problem is that airplane bathrooms are generally pretty disgusting and way smaller than your average bar or public restroom stall. Erego, there's absolutely no room to maneuver and the risk for personal injury is great (can you imagine having a soap dispenser jammed into your back? Uh-uh). Also, I can't be the only one who's on the point of nausea when I have to use one of these receptacles mid-flight.
They're SO foul. There's urine-saturated shreds of toilet paper and feminine sanitary wrappers scattered about on the floor. I'm more likely to scream out, "Oh god, don't let me get hepatitis, please!" than "Oh god, now, Please!"
Don't get me wrong, I've had sex in public places before and enjoyed myself immensely; more than once- in broad daylight (smirking to self). I propose a new mile high club: digital stimulation. No, not your cell phone, you geek.
If you have one of those cheap faux fleece blankets at your disposal, you've got a lot of wiggle room- especially if you're in a strategically located seat, which is easy to do since most airlines allow you to choose seats in advance. If you're not in first or business class (where there's a massive arm rest divide), you could do quite a lot under the protective shelter of one of those blankets without the other passengers knowing (pending back and shoulder problems).
My advice when traveling with a companion: Next time the gay air steward or AARP stewardess asks if you want an extra blanket, say, "YES!"