Mary Jane Watson aside, Gwen Stacy is the other love of Peter Parker's life. She's his age, she's attractive and she's intelligent - all of which seemingly make her ideal for him. But she also had sex with Norman Osborn - a man more than twice her age and the arch-nemesis of Parker's alter-ego, Spider-Man - and that's not so ideal. Moreover, she had a full on affair and the sexual encounter depicted in 2004's The Amazing Spider-Man #512 ended up getting her pregnant - evidently the Green Goblin doesn't like putting something on the end of it (and the creepy O-face pulled by Osborn was something readers will never forget). She gave birth in secret and the resulting children grew rapidly as a result of Osborn's altered Green Goblin DNA. It led to him having a hissy fit and throwing Stacy off a bridge as a result. This is not something people ever want to see happen in the Marvel Cinematic Universe. Or anywhere else, for that matter.