I think you might be taking Lini a hair to literally.

While I agree that she's kind of ridiculous, I think she's also prone to hyperbole and overreactions. Odds that Morgase was unaware of where babies came from at age 19 are pretty much nil, especially since she likely had the finest of educations available prior to her going to the Tower, and once in the Tower would have been exposed to this knowledge almost certainly. Morgase also showed that she was rather exasperated by Lini's attitudes as well, which just leads me to believe more that Lini's hangups didn't really have a material effect on a young Morgase. And while Elayne was certainly sheltered, she was also quite aware of how relations between men and women worked, if not exactly wise about how she went about them (i.e. getting knocked up on her first go with sex is a rookie mistake).