I believe the MILF definition refers to older women by age, so they don't necessarily need to be mothers.
Though Sara Bellum did usually act as the mother-figure in the original PPG cartoon, giving them advise and helping out when needed. I did remember a later episode where the Professor married Miss Bellum, but that was more a gag clip show as the episode was a "Remember when..." thing since the girls were cleaning out old stuff from their closet.
The gag for the wedding ceremony is that we never really saw Sara Bellum's face, and the animators decided to just make it Mojo when the hair was moved away.
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