This is one of those places in the bible that seems to be a stalemate because you have scripture on both sides of the issue. Jesus clearly said that those who would get a new body would not marry or be given in marriage and would be equal to the angels. This has led to the assumption that angels of all ages and all types could never have sex, which of course contradicts the story in Genesis and Jude and Peter, where angels had sex with human women and were also punished by God for doing so. The story about SOME angels having had sex with women is so clear that I cannot in good conscience pretend that it does not say that. My opinion, in the past SOME angels were capable of having sex and did so with women even having children.
Can't procreate if a person does not have the tools do it.Angels as spirit messengers have no form no DNA made up from the dust also things are created after their own kind.
That section in Genesis 6 is simply teaches the believers(sons of God) were being unevenly-yoked with unbelievers. (daughters of men ).