One such beloved ship is Buffy/Angel. The creepy elements are obvious: Angel is a stalkerish mass murderer who is over 250 years old. Buffy is an underage high school girl. So here come fans with the justifications.
One justification I've heard in more than one quarter: Angel has never been in love before Buffy, therefore he is emotionally immature, therefore he is her emotional equal, therefore, voilà: less squickiness.
First off, I think Angel did love Darla. Yes, his feelings for Buffy were very, very different, and very new, but I think he did love Darla. He certainly acts like it. And hey, in the episode Angel Darla says that she loved him and he loved her. Their avoidance of the word "love" is as much a retcon as the reveal of Dru as Spike's sire, and the use of Angel/Angelus to distinguish between souled and soulless Angel.
However, more to the point, even if Angel really had never been in love before and was completely emotionally mature in romance, the argument that this equalizes Buffy/Angel is highly disturbing and creepy.
Angel has a history of stalking, torturing, murdering, and raping young women and teenage girls. Buffy is implicitly and explicitly compared to Drusilla, the woman he tried to destroy. Look at this in a real-world context: Imagine an adult man who stalks women but has supposedly never loved one falling for a naïve teenage girl. That's an abusively unbalanced power dynamic.
And even if Angel weren't a monster, his age itself marks an enormous power imbalance between him and Buffy. Falling in love does not turn adult men into teenage boys. An adult man has experiences and power that an underage girl does not. Angel is sexually experienced, lives on his own while Buffy lives with her mother, and is at a different stage of brain development then her (assuming vampires have brain development…you know what I mean). An adult man has all kinds of legal rights that a girl does not.
And don't give me a line about how Buffy initiated the relationship and had "power". Buffy is a kid. Statutory rape isn't any less statutory rape because the victim consented. Angel deliberately and consistently hides huge parts of himself from Buffy. Buffy may think she's mature, but a lot of teenagers think that. Angel knows better. Angel had been around for centuries, and he's studied and destroyed a lot of teenage girls. Angel is a predator, and Buffy is too young to understand that she is his prey. Love and abuse and exploitation are not mutually exclusive.
Also, I'm forever mad at Angel for stalking a 16-year-old girl, getting into a romance with her, kissing her, sleeping with her on her seventeenth birthday, losing his soul and traumatizing her for life, coming back and causing her more grief and kissing her again despite the danger, insisting (in Lover's Walk) that he can't accept her leaving him, and finally leaving her for her own good because she's too young to make her own decisions…when she's finally a legal adult, finally graduating high school, finally coming into her own.