Leviticus 18:22-23
Thou shalt not lie with mankind, as with womankind: it is abomination.
Neither shalt thou lie with any beast to defile thyself therewith: neither shall any woman stand before a beast to lie down thereto: it is confusion.
 
This passage is pretty straightforward. God created them male and female for procreation. He joined them together in marriage as a symbol of his relationship with His bride, the church. He called any diversion from this model an "abomination". It wasn't until recently, as I listened to Pastor MacArthur teach these verses, that I realized that God placed these two verses together for comparison to help the reader understand how serious He is about this sin. When we think of a human being mating with an animal, disgust is the first thing we feel. God views this even more seriously than we do.
 
If there is any doubt about how God feels about men in relationships with men or women in relationships with women, look no further than Genesis 19, where God lays out in His Word the punishment that towns of Sodom and Gomorrah earned for their bold sin. If we don't understand how serious God was after reading that scenario, I'm not sure there is any more that God could possibly do to help us understand.
 
Marriage is between a man and a woman for life. Any deviation from this model is sin, and God will judge it.