I think it rates pretty low of "potential for harm" since the subjects aren't identified and they chose to sunbathe topless on a public beach. A photo where we have the subjects' permissions would be better, but I don't see how we could be sure of that (any kind of posing would ruin the photo - it would turn it from topless sunbathing to glamour modelling, a completely different topic). So I think this photo is appropriate.
Couple of (very) quick responses;
It's my view that the decision to sunbathe topless (or naked, or whatever) at a public beach does not equate to consent to publish an image to a wikimedia project. This may or may not be widely held - I dunno.
In terms of 'how to apply rigour' - I think the first stage is to agree whether rigour is necesary or not, and whether it's lacking or not (I'd say 'yes' and 'yes') - I have indeed suggested some specific ways, but the start point is to ensure the uploader submits such information - they don't currently.
The benefits of some sort of 'descriptive image tagging' to allow for greater project utility are a whole other kettle of fish - the idea of developing a 'sexual content' flag (or any other tag) is thoroughly rejected currently, for reasons which I believe relate more to the ability of the communities to engage in functional discourse more than the merits of the proposal (but hey - I'm an advocate, so I would say that, right? :-)
cheers,
Peter PM.