The answer to that question would be, God wanted to make a distinction between animal and man, who he had created in his own image. He wanted us to share a mutual bond of love and desire and above all, a spiritual union, that could be experienced by any other species, not even the ape who we, according to evolutionists, evolved from. In the animal kingdom, the female instinctively submits to her male counterpart as she enters a period of fertility.
1 Corinthians 6:15-20
"Do you not know that your bodies are members of Christ himself? Shall I then take the members of Christ and unite them with a prostitute? Never! Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, 'The two will become one flesh.' But he who unites himself with the Lord is one with him in spirit. Flee from sexual immorality. All other sins a man commits are outside his body, but he who sins sexually sins against his own body. Do you not know that your body is a temple of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore honor God with your body."
Quite clearly, human sexual relations were intended for love, not solely procreation as with every other species.
I don't think it is clear at all. What is clear to anyone who is not wrapped up in Biblical nonsense is that Homo Sapiens evolved from apes. Our DNA is almost identical.
And what evangelical Christians can't seem to get through their heads is that quoting from the Bible is not an argument. What is written in the Bible was is just fiction written by some people who were trying to explain the universe. We have come a long way towards answering that since their primitive attempts.