Well, you're 100% correct about the marriage being written lacking any imagination. That was in fact the problem with the relationship just prior to Gwen's death. Little thought was put into it.
However, it's pretty clear that Gwen could handle learning that Peter was Spidey. If she could handle being kidnapped by a giant monster and keep her cool she could handle Peter's double life. The issue of course would be her accepting that Spidey wasn't responsible for her Dad's death. And truth be told, had she lived that could have made for a great storyline- her mistrusting Peter and then having to come to terms with the whole thing. Provided of course they didn't go the soap opera route.
I know the likes of MadGoblin believe that her reaction in ASM #87 suggests she couldn't. But that's not really accurate. Gwen wasn't reacting so much to Peter's revelation, but the way he made it. Straight up, in that scene he was just acting whacko in general, ID reveal or not.
In the end, they're fictional characters. How they react or don't react is purely at the whim of the writers. SP is the prime example of this. If they wanted to have a Gwen-Peter marriage work, they would. But I'm saying to further my point made earlier that while MJ is indeed a great character, Gwen had more story potential for being a force in Peter/Spidey's life.