I was reading FC today, as you will do, and this paragraph made me go "Aha!" and think that Horatio himself was having to talk himself out of staying with her:

And this question of his never loving anyone; surely Marie was wrong about that. She did not know the miseries of longing he had been through on Barbara's account, how much he had desired her and how much he still desired her. He hesitated guiltily here, wondering whether his desire would survive gratification. That was such an uncomfortable thought that he swerved away from it in a kind of panic. If Marie had merely revengefully desired to disturb him she certainly had achieved her object; and if on the other hand she had wanted to win him back to her she was not far from success either. What with the torments of remorse and his sudden uneasiness about himself Hornblower would have returned to her if she had lifted a finger to him, but she did not.

He was thisclose to the edge; he could have talked himself into it even back then. Maria being pregnant was probably what swayed him; if Marie had been pregnant in LH, damn, he really would have stayed.