Why Did God Give Man A Penis?

Really, why? The bible says that God recreated the earth from what had become void and a wast land. He or she added the oceans, the sky and stars. He or she created land and filled it with animals and birds and put fish in the seas. Last but not least, he or she created man in his or her image. The story says God created man and it was good! God and the man spent time exploring the wonders of the earth and naming the animals. After some time, God changed his or her mind and decided that it was not good and that man was lonely. God put man asleep and took a rib and made woman. If woman was an after thought, why did God give man a penis?
Originally, man was created to rule the world without any women on it. It is unclear how God intended for man to reproduce. There must have been a plan that didn't include woman but did include a penis. No one seems to know what that plan was. Luckily for us that like women, the original plan was scrubbed and our penises now have a purpose other than writing our name in the snow.
I bring this up because of all of the anti gay rhetoric I hear from so called Christians. They say that gay sex is unnatural. Really? What do they base this on? The evidence in their own book would suggest otherwise.