God specifically says that it is wrong and why (I posted some passages that specifically mention this earlier although there are more –PM me if you would liked them for further reading). Throughout the Bible, God also speaks about the relationship between man and woman specifically (how to treat each other, love one another, etc.).
As 33girl pointed out earlier, the first chapter of Romans and the book of Leviticus will give you insight regarding “clean and unclean” sexual practices (this is what they are referred to).
Genesis chapters 1-3 explains the intent of God (the creation of man and woman for each other)
And of course, there are the obvious anatomical differences between a man and a woman and how we are made to “fit” with on another).
I hope this is what you were looking for S~n~S.