Having no goals or interests is pretty much standard for supporting characters at the time. I wouldn't say that Harry or Capt. Stacy or J. John Jameson were any more fleshed out. Mainly, you just saw them in relation to the main character. That said, giving Gwen to goal of "have sex with the father of one of my best friends" isn't much in the way of character development, or of positive, well-rounded portrayals of women in comic books.
When you mention Peter being non-judgemental, you touch on one of the problems I have with Sins Past. It's easy to be non-judgemental and forgiving toward someone who died years earlier. It would have been more impressive if it had turned out that Peter had known about it all along and had been willing to raise the children with her. That would have really been what a hero would do, as opposed to just not being a jerk. It would also have taken away one of my objections to Sins Past, that there was a period when Gwen acted as though everything was normal between her and Peter when she'd actually had the one night stand and then given birth. I suspect that if they had gone that way, though, the reaction from some wouldn't be "he was willing to raise Gwen's children as his own, how commendable" but rather "DUDE. Don't get played by a woman like that."
I'm with you on Marvels. I can understand why he'd remember her that way, but that panel made me wince. This is another place where denial comes in handy. The way I see it through denial vision is that Gwen Stacy was a great character who was written by Stan Lee, but was only used once after that, when Gerry Conway killed her. Other use of her by Conway? Nah, didn't happen.