The meaning of Genesis 6:1-5 is much debated. R. C. Sproul, Jr. provides as clear and simple an explanation as I’ve seen in a while:
There are several competing theories on this admittedly peculiar text, a few of them fantastic, at least one of them rather pedestrian, ordinary. Some suggest, for instance, that what is happening here is that angels, typically fallen angels or demons, are intermarrying with human women. My position is the far more pedestrian one, but one that carries with it an important lesson.
First, why I reject this more fantastic view. Angels, whether fallen or not, and though I am happy to concede they can appear in human form, are spirit beings. They have no bodies. Most of the time most of us remember this, though here some seem to forget.
Read the rest at Ligonier Ministries Blog.