There needs to be a reasonable level of explicit. /cm/ is more for tame pics that you could probably see being posted onto advertisements in public. /y/ is certainly much more risque picks. Unfortunately a lot is dependent on who posts and who sees it.
For example, two guys sitting next to each other without shirts on the beach? I would say is /cm/. But have one of them licking the other's nipple and that would certainly be /y/ territory for me.