This is a hard question, but I have some experience with it, since my gf likes prog.
Piper at the Gates of Dawn. Scarecrows and Gnomes can be fun, at a certain time and Sid's pretty tame compared to the crackheads in the music world nowadays. She'll think you're cute.
Moonchild. I think you know what I'm talking about. There's even a tranquil makeout section. Ladies of the Road's a little tough, you're not really a rockstar
Dark Side of the Moon, followed by The Division Bell.
And you and I is awesome. Also good is To be over, Wondrous Stories, and numerous Howe goings on. Guitar skills are sexy (so says she)
I actually listened to Eloy once (Mysterious Monolith) while making out but it was too much like some 80s trip out artsy thing and we had to shut them off.
Three Colours by Galleon is pretty awesome, that's a new one I just got.
Best prog pickup band overall? I would have to say: ELP. Still you turn me on, C'est la vie, Lucky Man, From the Beginning. They're all pretty chivalrous.
Gaston