I was just Googling my local porn theater and came across a website that had a list of the last remaining porn theaters in North America. Not the rooms with a couple of couches and a big screen tv, but actual movie theaters that show porn. I was shocked by how small the list is and happy that I had one of the few remaining nearby.
Dr. Emilio Lizardo's Journal of Adult Theaters: A Few Minutes With Dr. Emilio: T2 (Tornadoes and Theaters)
Here is a list of (still open) old medium to large adult movie theaters to the best of my knowledge:
Apex Theater, Baltimore
Twin Cinemas at Buford Highway, Doraville, GA
The Playhouse, Tampa, FL
Cinema Blue, Birmingham, AL
Foster Theater, Youngstown
Adult Cinema in Akron, OH
Cinema One, Chattanooga, TN
Paris Twin Cinemas, Syracuse, NY
Secrets, Oakland, CA
Oregon Theater, Portland, OR
Paris Theatre, Portland, OR
Metro Theater, Toronto, Ontario, Canada
Art Cinema, Hartford, CT
Strand, Kansas City
The Lido, Dallas (not really an old theater, but a large seating area)
Theataire X, Clarksville, IN