Sadly, I don’t think we’re going to reach a definitive conclusion here — the issue has very much to do with one’s perception of what is acceptable or family friendly, which is deeply personal.
At Postcrossing, we try to keep an open mind but have a few red flags for full frontal nudity, porn, or aroused genitals, among other stuff. Basically, if it’s not appropriate for daytime TV, then it’s probably not ok to show here.
It’s fine that members exchange these postcards between themselves (if they’re adults and interested in the topic), but these images don’t need to be posted on the website. This is the kind of thing that is most likely to get us in trouble with advertisers, and we depend on them to run the website.
A content filter or NSFW switch is an interesting idea… but at the moment I’m afraid this is not a high priority on our to-do list. I hope you understand.