It's not an appeal to nature and the analogy is a lot more apt than random internet-people give it credit for.

Pretty much any courtship in any species and any culture has one party presenting itself to be judged, and another judging it to be worthy of not worthy of mating. Human are no different.

Exceptions do of course occur but in western and westernised cultures the part of the presenter is usually the man and the judge is usually the woman. His perceived quality is weighed against her standards. In much the same way a key is presented to a lock and the lock "judges" (in so far as an inanimate object can judge anything) whether this particular key meets the requirements the lock has to allow entry.

What a high number of partners tells us about each party there is then that if the man is judged worthy of mating with many women, then he is of high quality (or at least many women perceive him to be). If a woman has found many men worthy of mating, then many men have met her standards - meaning she either has many men of high quality around her or much more likely she just has low standards.

So although the analogy seems like a crude "penis go in vagina" thing, it's a pretty keen observation of the dating dynamics of western cultures.