Bro Tim, a couple of thoughts...

1. You are projecting a requirement that it "must" be still possible, but keep in mind there may have been vast differences not only in the atmosphere but also in the physical abilities and DNA of early humans, for example in Gen. 5 we see that people lived 800, 900 years. That's a long time. Clearly this has CHANGED, and those life spans are not possible today—so who is to say that the interaction of Genesis 6 between the Sons of God and females did not change as well, perhaps God prevents it now? Either way I don't think we can claim it's a "necessity" to think that it can still happen today. We just don't know.

2. This is an interesting verse below, it would seem that there is a concern over how women dress and appear to angels, even in N.T. churches...

1 Cor. 11:10 "For this cause ought the woman to have power on her head because of the angels."

Matthew Henry:
"She ought to have "power," that is, a veil, on her head, because of the angels. Their presence should keep Christians from all that is wrong while in the worship of God."