There's a lot of room between the worst case and the best case, but even the best case doesn't make Gaiman look particularly good. At best he's ignorant of the messed up power dynamic in a relationship of an older man of his wealth and prestige pursuing a relationship with very young women (who in some cases are fans of his work). And it's hard to believe that a man of his age and of his intelligence is that ignorant.
I'm not a mental health professional, and this isn't a defense of his behavior, but I wonder if Neil didn't develop a bad case of Acquired situational narcissism:
Acquired situational narcissism (ASN) is a type of narcissistic personality disorder (NPD) that can develop in adulthood or late teens. It's characterized by narcissistic behaviors that develop after a person becomes successful or popular, and can be triggered by fame, wealth, or other forms of external success. ASN can also be influenced by the enabling behaviors of society, fans, assistants, and tabloid media, who may reinforce the idea that the person is more important than others.
The lack of social norms, controls, and of people telling them how life really is, also makes these people believe they're invulnerable, so that the person with ASN may suffer from unstable relationships, substance abuse and erratic behavior.
In a lot of ways, to a certain segment of fandom, Neil was more like a rock star than a typical "comic book writer." I wonder if the extremely atypical level of fame he received when Sandman first came out coupled with the last 30+ of years of getting his butt kissed didn't warp his worldview and allow him to delude himself that his bad behavior was welcome.