I don't think anyone is overreacting here, at all... especially not Dora. It's a sad, sorry thing, but the truth is, her entire life, Dora has watched her older brother fuck around with her friends, and considering what he did to Faye... no, she isn't being a Crazy Bitch, she's being a rightfully protective friend, saving her socially-inept* friend from the same horrors others have had to deal with.
*And while, yes, she won't get any less inept by their not allowing her to socialize... it would be nothing short of disastrous to let Sven do this to her. She can barely cope with the fact that her roommate doesn't want to kiss her - what would she do if she had her first time with Sven? And if he - as he seems to do a lot... - slept with another woman (Gina Riversmith take two?) it would utterly destroy her.
However, if she was to find a nice geeky dude who is not already interested in someone else and not, you know, sharing a home with her, I'm sure they would all encourage her to go for it.