Refers to our spiritual and mental condition, not physical. We don't look like God in any way. It just means that we are able to make logical assessments, like God, and moral choices, like God. It also means we are eternal beings, like God. That is how we are in his "image". Even still, I suppose that could be taken to mean that an eternal, moral being should not mate with a mere animal. Scripture does not say that, but it's fair speculation. It does not, however, apply to homosexuality, which is still a relationship between two people.