Another vote for telling the DC not to get into their bed in the mornings, if you worry about it. Send a nice big cuddly toy with him to snuggle up with when he stays with them.
I've never kissed any of the DCs on the lips, mainly for reasons similar to those Teaandcake mentioned. I wasn't worried that any of them would kiss random people they met on the lips, but so that they would think it a bit off if someone else did it to them while children.
I cheerfully strip off in communal changing rooms and fitting rooms where there's a certain formality about everyone being naked and everyone is minding their own business. But I closed the bathroom door and made the DCs knock when they were small and they wanted to talk while I was occupied there. I felt it was better to start as I intended to continue and not change the rules as they got older, just get them in the habit of considering other people's privacy (and maybe their own) while they were young.
I think it's rather sweet that the gps sleep naked, and would not be inclined to rush to put on pjs myself if the DCs climbed in with me. (ExH on the other hand would never appear naked in the presence of the children at any age, no matter what the circumstances. Never got to the bottom of many of his hang-ups, including that one.) I'm pretty sure all my DCs' gps sleep or slept with pjs on, and DCs were never babysat by any of them overnight, so the issue of climbing into bed with them, naked or not, never arose. I think if they were ever staying over I would tell the DCs to stay in their own beds or play quietly in their rooms and wait to be greeted there instead of barging into someone else's bedroom. The gps are entitled to their privacy too, and might even be enjoying a nice snog or even more themselves first thing in the morning.
Mind you, my DCs were more shocked by the sight of granny's teeth in a glass by her bed one time than they might have been by any hypothetical sight of her without a nightie on.
I don't think you can bring up the issue of the DS being in their bed with them naked without coming across as accusing them of bad intentions at worst or reprehensible carelessness at best.