DarkLady, what a fascinating idea about Roxanne! I've tried in vain to figure out how she became a vampire if Barnabas wasn't let out of his coffin, but this gives rise to new speculations. Maybe after just one nursing session with baby Gregory she decided the answer to her problems lay in the supernatural.
But as to Zahir’s original question, we must keep in mind the statement of Reverend Trask (1795 Trask, that is) in episode 434 that he had not experienced physical love. My previous explanation of how Reverend Trask had managed to become the father of Lamar Trask was that Reverend Trask must have adopted the son of an identical twin. Zahir’s idea, however, gives me a more plausible explanation: when Reverend Trask disappeared, his wife was consoled by his identical twin brother, and they ended up in bed together. The fact that Reverend Trask had refused to fulfill his marital duties in bed would have made Mrs. Trask all the more willing, perhaps. And then, of course, she passed off Twin Trask’s son as Reverend Trask’s, and was exceedingly thankful to the Almighty that Reverend Trask never showed up again to ask inconvenient questions.