It has occurred to me that Christ's pronouncement that angels don't marry or procreate "in heaven", might not apply to fallen angels, who no longer veiw God as their authority. Christ only says that angel don't marry/procreate. He doens't say they can't. There is a difference.
I still prefer the Seth/Cain linage theory
though that fails to answer the question of the giants.
First, there were giants in the land prior to this event and there were giants in the land after the flood, Noah and his children, who were of the line/lineage of Seth.
Also the term, in heaven, refers or give specific connotation to them being spiritual beings, like the angels are. This is important to understand since Jesus was speaking to the sadducees who not only did not believe in a literal bodily resurrection but also denied the existance of any type of spiritual existance of us as well as any type of angelic being. Therefore Jesus reference here is specifically dealing with two issues at once.
However the 'in heaven' does not relegate the specific nature of spirit being only to heaven, since demons are fallen angels then the nature of their being is no different than that of the angels of heaven. Therefore in understanding that Jesus was dealing with the nature of the their being as well as ours at death, we also know fallen angels are of the same nature (in which they were created) and being as all other spirit beings.
Another point I would like to address about these fallen angels doing what they want because they do not see God as their authority anymore.
Satan couldn't even
tempt Job without God's express permission and not only thatwe find no where in scripture that they do not respect nor adhere to God's authority anymore but in fact we see the very opposite except that they were cast out and awaiting their eventual judgment.
Remember that scripture tells us .. the devils (demons) believe in God and tremble.
Therefore let us not forget that demons DO see God as their authority. He is not an authority they willingly obey, but they do obey His every command.
Thus the point of Jesus statement was two fold. One is that Jesus was dealing with spirits beings being real or existing. And two.. was speaking of those things (not precreating, marrying, or given in marriage) pertaining to those beings in heaven (like the angels - all of whom were created in like fashion) and thereby by extention includes those fallen angels (also angels from heaven) who were created just like the other angels before they fell.
Remember, this is not a command in heaven which they can not do, this deals with their very nature and creation. God states that all things procreate after their own kind, but angels do not procreate themselves, according to Jesus, thus marriage has no meaning in heaven and is the very reason there is no marriage nor given in marriage since it's main function is to create life/another being in a God honoring way. Angels are only created by God Himself.