So, speaking personally a lot of it is just general mom-complex stuff. Marge is the best example of this. Loves her family, loves her kids, loves hard cock, classic mom stuff.
As for Lois and Francine, the show treats them as cheesecake objects a lot of the time, which makes it easy. They're a lot worse at being moms, which makes it more likely they're on the crawl for young dick, too.
MacFarlane art is just good enough to get the point across most of the time, I can't claim it's the thing driving my love for these characters but it works when it has to.
Also, I'm a sucker for nasal voices, which, check check and check.