Is that the one you're going with? How about boy-on-boy? Or most of the rest of the world, where breasts appear at the park and in other definitely non-sexual contexts (breastfeeding happens in public here, although in good old North America, it's still a bit contentious, for whatever reason)?

I personally find it rather non-trivial defining these kinds of things ("relationships" too, not to mention the general vagueness of that work, since I have some kind of relationship with my banker, say)... I've heard something like "when something you wouldn't want your girlfriend to do with someone else", but I find that one to be no good for me. :-P