The first time I heard something about this (although at the time and relate it) was in the legendary movie "American Pie". In it are having a party at the home of one of the protagonists and some kids see the photo of the mother, and brand it as MILF (Mother May I Follaría). I even well after I thought it was a word (acronym) invented in the movie.
The acronym comes from MILF Mothers I'd Like to Fuck, that is the English translation of the MILF (though I give my Spanish version has been that the "copy" of the English original 
.)
This is a genre where the protagonist is a woman, usually a very attractive mature woman, where is the incentive or morbid question of a mother. They play with the curiosity that gives the status of "mother", just as it gives the status of "married". Let's face it, to you who gets more? A "free" woman or married? For this reason
Writing this post, is not so much as to enlighten you on the subject (this'll be sure you more knowledgeable than yours truly), but by the surprise that the other day I discovered a range within these films, featuring only women. These are young girls who sleep with the mother of his friend, or neighbor. It is still an interesting variety 
And apparently is called "Lesbian Milf".
Any of you to seen some of this guy? Do you think? It really gives you more morbidity that the girl has a special conditional (married, neighbor, mother, etc..)?
As long as I can, some additional information as links:
- Official Website MILF
- Website about Lesbian MILF
