The Navarre Bible Commentary gives a good explanation of this passage:

Alluding perhaps to a myth in vogue in ancient times, to the effect that giants were the offspring of women who mated with beings of higher order, the sacred writer is stressing here the headway that sin and disorder have made. In this passage we are told that God put a limit on man's lifespan as a punishment for sin.

We cannot quite work out what the expression "sons of God" means here. Jewish tradition and some ancient Christian writers read it as meaning fallen angels; but that explanation does not fit in with the spiritual nature of angels. Therefore, it has been interpreted as mean good men, the descendants of Seth who indiscriminately took as wives descendants of Cain, here called "daughters of men". This is the explanation given my St. Augustine (De civitate Dei, 15, 23), St. John Chrysostom (Homiliae in Genesim, 22, 4; St. Cyril of Alexandria (Glaphyra in Genesim, 2, 2) and other Fathers. Mankind's moral decline on account of pride and abuses of marriage prepares the way for the upcoming account of the flood.

It is interesting that today, in the 21st Century, mankind has morally declined on account of pride and profound abuses of marriage (vis-a-vis, domestic violence, divorce, abortion, incest, fornication, adultery, concubinage, etc.)

Bottomline: the most logical and probably meaning of this passage is that the term "sons of God" refer to the descendants of Seth, and the "daughters of men" refers to the descendants of the evil family line of Cain. The good family line of Seth, corrupted and began to marry from the evil family line of Cain. This would have happened only with a societal moral decline.

As a result, the world became so evil that God destroyed all the earth except for Noah and his family.

As for the references of angels in bodily form, yes, angels can, when God wills it, appear in bodily form. This is in appearance only. Angels do not become biological creatures.

God Bless,
Bro. Ignatius Mary