I'm pretty sure I was the one that started the other thread
The problem was never resolved, even by restarting several times, waiting a while, etc, and I didn't want to reinstall my game just because of it. I deleted the file and thought no more about the problem.
Recently, it's come back-it happened with two different couples. I still moved in one guy as a friend, though, yet it still didn't work (he was flirty, as well!) Ever hopeful, I moved my sim and him to another house together, and everything is as it should be now. I think I'll try with the other couple, if the girl is still around.
Hopefully, this will help!
Oh, and by the way, they can still Try For Baby; just get them both lying on a double bed and the option, along with a few others, should come up. It will still say that 'Sim X thinks Sim Y is ok', though.