She is too close to aging up to become pregnant. Her household is full and you have NRaas > Woohooer > Try for Baby > Species:Human > Maximum Occupants for User-Directed set to something other than 0. But the first thing that comes to mind is that your sim is being denied the chance to become pregnant herself, not that it's because of the gender of the potential partner. Sometimes these puzzles fall into the 'Things Are Not What They Seem' category.
