In the 1970s there used to be porn movies shown in theaters (in France if I'm not mistaken : in regular theaters then in specialized theaters after regular theaters were prohibited from showing them). You had to sell tickets etc and I suspect this makes sense only if the movie lasts some length. People won't bother coming to a theater, get in line for a ticket etc. for something that lasts 5 minutes. If you have short clips you must at least market them as one compilation.
That is right, there were such 'sex' theaters back then. But if you now look at the material shown, it wasn't really porn. There were lots of naked breasts and bottoms, often pubic hair which is the same as showing nothing, but no explicit scenes with erect penises or spread open legs and vaginal slits. It was exciting for the times, but not porn as in the illicit material or what we now know.
I went to such theaters a few times when in university in the early 1970s. It was disappointing stuff, even back then.
That said there were some very successful series back then like the German produced 'Schulmaedchen Report', with at least 13 movies in the series. But when you view them now, they are cute and amusing, fun-loving, but not really arousing at all. That is why on porn sites, such material is usually classified as 'soft porn'. Sort of like a tad more explicit David Hamilton movie (like Bilitis, Tendres Cousins, A Summer in St. Tropez) but a tad more vulgarish in a way. Lots of promise of sex in the air, but little really ever delivered to the viewers. Or movies like the prize-winning compilation 'Pornografi'.
And come to think of it, movies like the 'Schulmaedchen Report' series and certainly 'Pornografi' were really nothing than a series of short scenes, many bearing little continuity to the next.
This also reminds that not so long ago I tried watching the big 1970s hit 'I Am Curious (Yellow)'. Talk about much ado about nothing in the way of sex. Absolutely boring and a few nude scenes. But a big hit back in the 1970s.
But anyway, you are right about the sex theaters, they had to offer something of considerable length to justify entry price.