I'm having a hard time determining if Molag Bal is having six with the soon-to-be-vampires, or if he is having six with someone else and the child is a vampire.

His realm being 'Coldharbour', a Daughter of Coldharbour sounds like someone that is born there.

It's figurative, not literal.

It's a rebirth. Son/daughter is used to convey a symbolic relationship a lot, both in real life and in the Elder Scrolls. Besides, Serana tells us straight up that she and her mother were '[censored]' by Molag Bal. Both are Daughters of Coldharbour.

http://www.imperial-library.info/content/vehks-teaching

The women he, uh, brutalizes become vampires. The reason why he treats women this way while letting men pass simply by sacrificing innocents is the Female Principle, or the Womb, if you will. I don't think they literally give birth to monster-babies, though.

Let's stop beating around the bush here.

targ says:

It's figurative, not literal.

Is Molag Bal having six with these people or just cursing them?

I think he does have six with them in a ritual coupling that's meant to reenact the corruption of Lamae Beolfag.

The major difference here is that the women seem to actually be consenting to it.

http://www.uesp.net/wiki/Lore:Molag_Bal

That might help.

Molag bal, the king of swing.