Just a small example of the vastly different and complex systems of reproduction:
For the movement of animals and plants from an aquatic to a terrestrial environment, the method of fertilization had to be changed. With plants, sperms are carried in pollen to the flower so that fertilization will occur internally. Earthworms gametes are exchanged when bodies are joined in a wet environment. Other invertebrates like terrestrial mollusks are hermaphroditic. Internal fertilization is the method that is evolved in most other animals. In birds and some reptiles there is a cloacae or receiving chamber for sperms and there is usually a small penis. Rattlesnakes have a forked penis and can mate with more than one female at a time. In mammals and some invertebrates like insects and spiders there is a intermittent organ (penis) that is inserted into the female. Internal fertilization therefore occurs in a liquid environment.
Now to the human penis. You have brought up the dreaded "why" question. I get these from my students all the time. We cannot answer "why" questions directly. All we can say is that the penis is like it is because, through the evolutionary process it has developed in the way that works best. (The "why" question is the only question in science that we can answer by saying "Because, that's why” ). The size of the penis is related to the size of the animal and the position of the female organs. (The penis of the blue whale is 5 meters (16 feet)long.)
As to the "active" male role in reproduction I am up in the air. The fact that some mammals like cats and dogs have a estrous cycle with the female receptive to the male only a few days out of the month obviously has a survival value. Why we do not is a mystery except that sex in humans has emotional rewards and with cats and dogs it is a rape.
The highly complex sexual reproductive systems we observe in nature need a better explanation than “because, that’s why”. When I‘ve questioned in the past as to how male and female reproductive systems co-evolved (morphed so precisely), I‘ve been told, “When you say morph so precisely you imply you believe that there was a plan that had to be followed. There is not.” Obviously, in regard to evolution, there is no “plan” to follow. The co-evolution of male and female evolving these highly complex systems occurred just "because they did" (like everything else). The personal credulity of evolutionists is just so overwhelming that it boggles the mind.
Consider the genitalia of various waterfowl:
From the article:
"So, the twists in the oviduct appear designed to exclude the opposing twists of the male phallus. It's an exquisite anti-lock-and-key system."
The number of sacs and spirals in the reproductive tract of various female waterfowl correlates strongly with the length of the male phallus. Comparing the phallus size and oviduct shape in 14 different species of ducks and geese, the authors show that the genitalia of males and females have dynamically co-evolved with one another.
There’s that dratted word design again, but not to worry, there are unlimited just-so stories that one can consider in order to try to simplify the vast complexity of these reproductive systems that evolved through the evolutionary process. It’s not a stretch to consider the microevolutionary means in which organisms adapt within their species, but really stepping back and considering this all occurring from that first spark of life is asking far too much from matter alone.
Casey Luskin recently linked to a portion of Getting the Facts Straight: A Viewer’s Guide to PBS’s Evolution which is relative to this topic. Episode 5: Why Sex starts on pg. 69 of that document...a very interesting read.
Oh, and here’s a little just-so story that I wrote a while back in regard to sexual reproduction and the Big O.
Enjoy...maybe I’ll submit it for peer review.