I dont know why people assume Molag Bal and Serana did "it".
Because Molag Bal is the Lord of [censored] and Daedric Prince of Domination. And it is heavily implied that they did "it". One, because females were offered to him by vampiric tradition, and Molag is known for his infamous sixual history with Vivec and among other things. Secondly, when asked on how she became a vampire, Serana specifically mentions Lamae Beolfag and how she was an "unwilling" subject while Serana had no choice but to comply to the "degrading" ritual.
What, did you think Molag just squats and produce feces all over Serana to turn her into a vampire?

. Tradition of the pureblooded transformation comes directly from the first source, which is Lamae Beolfag, and she had to undergo a sixual molestation before she was truly turned.