Starting in the late 1960s there was a subset of films & magazines that presented themselves as porn, but stopped short of actual penetration - sometimes simulating it but masking the lack of sex with clever camera angles or body positions. The rationale being that in locations where true explicit porn was technically illegal, these could be produced, bought, & sold with no risk.

Sounds boring - why go for the fake stuff when real porn is available? Thing is, many of these images are pretty damn sexy. Some of the couples look legitimately turned-on (there are accounts of avid dry-humping during these shoots) and often the girls were aesthetically a step up from the smaller proportion of women willing to actually prostitute themselves on camera. So in the best of these you have pretty girls (and guys) who look like they really want to fuck and getting very near the act, with the principle of Less Is More and the sense of anticipation giving them a genuinely erotic feel.

Your mileage may vary, of course. Step into another gallery if double-penetration closeups is on your menu. ;)