Clearly the arguments that sound less convincing have failed to survive and reproduce. The existence of evolutionary arguments is therefore proof of the correctness of the theory! ;)
Except penises come in quite a variety of shapes across species.
Distinguishing actual driving traits from those that just happened to ride along (i.e. men's nipples and other various vestiges, bright red colour of foxes where neither the foxes themselves, nor their predators or prey can see colour), is very hard.