First, I am not defending the view that (fallen) angels directly corrupted the human race in Genesis 6. There is a reasonable argument to be made though, that they were indirectly involved (through possession of men).

I have heard this argument before ... using the two texts in Matthew and Mark as "absolute proof" that angels were not involved in the Genesis 6 corruption. However, Jesus only says that the angels in heaven are not involved in marriage unions. I think these texts are far too weak to hang the label of "infallible proof" against the argument put forth in the Genesis 6 context.