I honestly think Sakura cannot have children for some reason. As it makes no sense that Sasuke wouldn't have a child with his own wife(he acknowleges this). In which case, I can see how Sakura might have suggested that sasuke have a child with someone else. Or maybe Sasuke suggested it. Doesn't really matter. After all, Sasuke always wanted to revive his clan, it would be a sad thing if he couldn't do that simply because his wife couldn't get pregnant. Karin would be the logical choice, as she is the only other female in the manga, sasuke has been shown to care about(to an extent) and also because of her clan.
If this is the case, to me, this would probably be the biggest win for SS shippers. Initially, a lot of people thought he married Sakura simply out of the need to revive his clan. But with this scenario, it shows that he actually learnt to love Sakura. Despite the fact she could not get pregnant, Sasuke still stayed with her, when he could have had almost any girl he wanted as a wife(karin included).