They didn't think of the further implications of the time travel paradox
(Spoiler for Episode 6, Season 2)
Claire persuades Jamie to cancel the duel against Black Jack Randall, and wait at least a year to kill him, until his child, Frank's ancestor, is conceived, in order for Frank to be born.
She justifies this by saying that she doesn't want to kill an innocent man, Frank, by preventing him from being born, and she has a hard time to convince Jamie about that.
But she didn't think of a much better argument, at least by Jamie's point of view: If Frank is never born, then he can't marry Claire, and not take her to Scotland and she would never go to the rocks and pass through time and never meet Jamie. So she could tell Jamie that if Jack Randall doesn't get a son, then she wouldn't be with him.
This would have been a much more persuasive argument for Jamie, don't you think so?