It is true that God intended us for monogamy, but it is not true that we are not animal. "Animal" simply means an animated biological creature, which we are most definitely, as opposed to inanimate creatures like plants. We are not mere animals, because we have a rational soul, whereas mere animals have sensitive souls.

God made us out of the same stuff. We have a lot of the same instincts, but we have the mind to guide us in decision-making. We are natural beings with natural needs and inclinations, but we are called to the supernatural, achievable by the grace of God. Without the grace of God, we revert to animalistic behaviors, which we see evidenced in the above discussion.