Ok, people enjoy it because it makes them feel intimate with each other,the act of intimacy arouses sexual desire. Is that enough or is the studying ancient cultures part mandatory?
The oversimplified brand of logic is just you generalizig my disagreement to support your brand of over analysis.
Please, take as much closer look as you want but do look at the whole picture,not just the booky part. Ask yourself why you enjoy it first,ask why others enjoy it and start there and stop when you have enough to adequately explain it.
I stand by my statement, a psychological need for intimacy is the root of the desire. It could be licking each others ear lobes or something that would now sound weird, the act is merely an expression of intimacy which is to know and to be known.
I would even say your conclusion can be viewed as an "oversimplified brand of logic" as you put it since you are not explaining why feeding chewed food is tied to sexual expression of affection and why it is prevalent strictly in a sexual relationship and why parents don't make out with children or between close friends and siblings,when adopted why did it become sexual? And to be clear a peck on the surface is not what we are talking about here since that is not exclusive to lips (forehead,etc...)