The bible shows that those angels who did this were banned from their positions in heaven, and they were put in a state different to what they had before, it explains a state called Tartarus. Since the flood, no, demons can't mate with women, as the angels who forsake their original position will be also put in this state.

Well, this question assumes that I fall into the 'stars of heaven/sons of god' were angles camp. There are several ways to interpret what that means. Personally, I find it hard to believe that beings described as being spirits elsewhere could do human women. Just my take. I think it more likely that we are talking about some variety of supermen if anything.

This is assuming that the "Sons of God" were angels, yes? I don't think that's ever explicitly stated in the standard Bible. It may be in apocrypha such as Enoch. Are the The Nephilim who were on Earth after the flood assumed to be survivors though, or were they new offspring? The Bible does say that they ". . .were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them."

The Bible never ever says any such thing. Many have interpreted Genesis 6:2 to imply that such a thing is possible, but that's just one highly tenuous interpretation, and nowhere else does Scripture even suggest or imply that it's possible. That interpretation also ignores/is blind to the fact that Gen 6:1-8 is an allusion to a recurring theme throughout the Torah, that the people of God (Israel) would be corrupted by intermarrying with Gentiles, especially Canaanites (the people of the land/earth). In the wider context of Genesis, and the epic/lyrical structure of the work, it is clear that the story is one of the 7 refrains of Genesis (the other 6 being the stories of: Adam, Even & the Serpent; Abram, Sarai, & Pharoah; Abraham, Sarah, & Abimalech; Isaac, Rebecca, & Abimalech; Jacob, Dinah, and Shechem, and finally Judah, Tamar, & Shua/Onan -- all of them variations on the central theme of the Torah: the ongoing saga of God, Israel, and the world). Each of these vignettes is followed by the exact same theme/story that Gen 6:1-8 is. The future generation of th children of God (the righteous) are at risk, God acts decisively, you have a judgment, a "coming out", a promise, a blessing, a sign, and a commandment/warning. Each of these is in turn followed by a story of how the rightful heir (according to the world's estimation) is passed over for the heir and covenant bearer of God's choosing: Seth instead of Cain, Shem instead of Ham/Canaan, Abraham instead of Haran/Lot, Isaac instead of Ishmael, Jacob instead of Esau, Judah and Joseph instead of Reuben, Simeon, and Levi, and Manasseh instead of Ephraim. Given the place and function of Gen 6:1-8 in the structure of the Torah, it's evident that "the Sons of God" refers to male descendants of Seth while the Daughters of the Land refers to the female descendants of Cain.

Such things never happened. Stop being such a tool.

Well, I can tell you about a real "angel" who used to mate w/ a mere mortal well after the deluge = me! ;-)

yes, how else would you account for all these single mothers?

Yes. I mated with an angel. A wonderful wife and mother.

After the incident described, they are no longer permitted this sort of activity...