The 'FG/AC universe' ? What do the acronym mean and stand for?

Dragon x Griffon = Fluffy/Feathered Dragon or Scale armored Griffon?

The other reason Spike didn't make a move was because Twilight had specifically told him not to have sex before marriage because she didn't want to have to deal with whatever babies a young horny dragon might produce if he decided to have some casual sex with whomever was up for it, not aware of what species other than dragons he might genetically compatible with. The main other reason Gabby didn't make was because... well, although she never showed it, she was a massive closet slut and happened to have a thing for adorable and naive barely legal boys, boxes which Spike ticked very nicely. Sooner or later however, all they needed was a golden opportunity... quite literally!

Twilight never hear of protection and sex education?

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the FG/AC universe stands for the "Fluttershy's Guide/Applejack's Club" universe, also if Gabby did get pregnant I imagine the baby would probably come out as a mixture of lizard and bird, like an archaeopteryx https://upload.wikimedia.org/wikipedia/commons/a/a4/Archaeopteryx-model.jpg

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Twilight's a princess, she doesn't have time for all of that! :P