Robert De Niro gets his knob polished in Bertolucci’s 1900.
Well, it may not qualify as mainstream exactly, nor are the stars household names (at least here in the States), but the 1976 Japanese film In the Realm of the Senses (Ai no corrida) was explicit but artistic and widely acclaimed.
There’s also 1982’s Café Flesh, which I saw in its first run release in an art house theater (not a porn theater–really!). IIRC, it is a fairly interesting post-apocalyptic sci-fi pic in which people are either Sex Positive or Sex Negative. Negatives (99% of all survivors) get sick if they have sex, so they come to Café Flesh to watch Positives perform. The performances are hard-core. Richard Belzer has a small role as a heckler.
Also, there was a brief time in the early 1970s when porn almost became mainstream, thanks to a confluence of events (the abolition of the Production Code and introduction of the MPPA rating system, which allowed producers to self-rate films as “X”) and, in no small measure, to Deep Throat. Sadly, this enlightened attitude didn’t last long.
Umm… this post isn’t going to give me a reputation as an expert on porn, is it?