Everyone you've ever met has kissed their kids on the lips in front of you? Or do you ask each person as you meet them? Personally, I can't remember ever having seen a parent kiss his or her kid on the lips in real life.
For us it would be about the same. We definately have never thought about other people thinking it would be weird to smooch our sweet little one on the lips from time to time, I kiss her cheeks, her belly, her little hands and feet....
And all of my friends are fine with it and I have seen them all do it, too....
But I could see other people having rules about such things, people have rules and hang-ups about all kinds of stuff.