Yes he can. She hated having six with that limp bastard. She even admits too that
When does the game ever even IMPLY that she had six with Molag Bal? The game says she was "offered" to Molag Bal, but it was implying that as sacrificially not sixually, unless I completely missed that one.
At any rate, I support the notion of being able to marry Serana. When Bethesda goes around to fix the buggy mess that this DLC is, they should make it so we can marry her as a peace offering to us. I mean, she is the most endearing character in this entire game and she basically makes this DLC as good as it is, so I think it is the biggest slap in the face when they tease the possibility of being able to marry her, and then nothing comes of it.
All things considered, I feel like this whole situation might be one gigantic joke. If it were anywhere near April right now... O.O