Decline, decline, decline. I've done it once, well more than once, but once with the same sim, and with the aid of mods, I was able to progress the relationship. The problem was the children looked absolutely abnormally hideous. I was able to cheat and get their faces to look normal at least, but their ears were extremely tiny, and it was not something I was able to ever fix. Everyone was stuck with longer hair, even if I wanted short hair on either the boys or girls, they all needed longer hair to cover those ears. To picture how small those ears were, just picture how small the noses were on those Sims 2 aliens. No matter how many generations had passed, those ears stuck, I couldn't breed the ears out.
