I think it depends on the couple's preferences for having babies whether to "too soon" message comes up or not. If they both want babies, the interval will be a lot less than for a couple with "no way" in their preferences.

As to the "not sure if they're ready" messages, just keep trying, and eventually they'll kiss and make their way to the bedroom. Just be sure to look out for the message though, as the arguing sound effect carries on for a while even when they are on their way to make a baby.