DP feels it's inappropriate for male relatives to kiss grandchildren (our kids are both girls, ages 1 and 5) on the lips. As a general rule of thumb, I would agree. If you asked me, "Should male relatives kiss girl children on the lips?" I would say no. However, I don't feel like it's a big deal for this one relative (he's the only one who does it) since it's just what he does, and I've seen him doing it all the time, everywhere, for 40 years.
This has come up recent because DD1 likes to kiss people on the lips (other grandparents who are caught off guard by this, babysitters, etc.), and we are working on helping her learn that she is only allowed to kiss her mother and sister on lips, not other people (DP would prefer not to have her kiss him on the lips).
My plan is to say to my dad, "We're trying to each DDs appropriate boundaries about kissing, so I'd like to ask you not to kiss our kids on the lips while she's learning about this." I think he would say OK, but that he'd have a hard time remembering since it's his habit, so I will need to mark my calendar to email him a reminder before we see him. (We live 4+ hours away so we only see him about 2x a year.)
Once DD has internalized the "regular" rules about kissing, I'm inclined to let it go. I think enforcing the, "You can kiss me and my sister and everyone else but don't kiss my kids" rule would take a LOT of reminding (every time we see him, for all the years to come), and he would feel hurt, as if I were accusing him of sexually abusing his grandchildren. I can live with all that -- it just feels like a lot of work to me. So I'm inclined to let it go and let the kids learn that Grand-Pop is a little wacky in his affection, the same as the rest of the family has learned.
DP feels that it's a double standard to teach our daughters not to kiss people on the lips, but not to expect the same of my dad in terms of his granddaughters.
We could use some outside perspective!