Why a creator, god, nature, (or whatever floats your boat) has chosen "unclean" organs for reproduction? What was the philosophy behind it? Perhaps he wants to show how unimportant and filthy you are and even then your are proud of yourself. But "unclean" is merely a human value judgement. The universe doesn’t care about such labels. If you seek to understand the universe you have to toss out the human concepts first.