Okay, so I had an idea today. If you’ve ever wondered how the heck your Undertale OTP would kiss because it doesn’t seem physically possible, this might be relevant to your interests.
We know that, among monsters:
1) body configuration varies enormously. Not all monsters have lips, mouths, noses, faces, etc.
2) regardless of body configuration, every monster’s magic, soul and body are tightly entwined, and
3) nose nuzzling is a competitive sport apparently???
Considering all 3 factors, I propose that monsters don’t kiss as we know it. Their kissing/nuzzling is more about touching their magic and their feelings together and seeing what happens. And sometimes, the result is showy.