I did stop my PIL's from kissing my children on the mouths. I've never done it, never had it done to me. Not a sexual aversion, more of a "Yeuch" one.
My FIL had no teeth, was a very heavy smoker, and his mouth was extremely heavily nicotine-stained (dark orange) his breath absolutely stank. Now why would I allow him to kiss my brand-new, shiny pink sparkly baby on the mouth? No, no, no, no, no. Absolutely Not in this World, or any other. He started off doing it, but I did have to put a stop. Mouth infections are passed on this way (even if a cold-sore or mouth ulcer is not present at the time, they can lie dormant). Plus - why should she have to endure a sloppy, smelly wet, icky kiss when she has no say in the matter?
My poor Mum, on the other hand, had a Grandfather who used to kiss her on the mouth when she was a little girl (with full tongue). She objected, but her Mum just said, "Oh, it's just because he loves you, darling" and she had to endure it for years (and more). That was a sexual thing, and she was made to put up with it.
Not saying both types are interlinked, but both are unpleasant, in different ways. Lots of adults kiss children on the mouth - I personally think it is icky, for lots of different reasons. Mouth-to-mouth contact is either for lovers, or for life-saving purposes (and even that is done with protection, where available).