Hints (some of which are seemingly obvious) that the Tumblr site your looking at is more about “sex” than true Nudism.
1) Only 1 Gender is seen or portrayed.
2) Sexual acts are portrayed (I don’t want to describe them here – but you know what they are.). These sights I would label more as people who like sex in public or who are (in a possibly dated term) “Swingers.” Both of which,in my view, are fine for Consenting Adults, but a true Nudist Resort or Nudist Beach is not the place for this behavior.
3) Only 1 age group (usually young) is seen or portrayed.
4) Only 1 body type (usualy thin) is seen or portrayed.