Sometime during the last months of the Clone Wars Anakin and Obi-Wan are exposed to sex-pollen etc. and end up having sex (great sex at that)...Obi-Wan gets pregnant, but Anakin does not remember the entire episode.

Obi-Wan has always felt strongly about Anakin, but he's also always known about Anakin's marriage and so declines to inform him of his impending fatherhood. Cue absolute angst for Obi-Wan,and a happy ending.

+ If Obi-Wan had been a virgin.
++ If Padmé has never been pregnant, but is very interested in OT3 and so is Anakin's willing partner when it comes to Obi-Wan