How does remembering their first marriage negate the memories of having sexual encounters with other people? It's the same issue...they never officially annulled, so logically they were still effectively "cheating", best way to throw that under the rug is have the characters admit what we know: from what MJ said in OMD she knew any person they became would be due to Mephisto's influence over them, so they were essentially not acting in-character, they were different people and that was of deliberate design
But if we follow Spencer and Slott's line of thinking, Gwen already came back in Clone Conspiracy, and she told Peter she always figured she'd lose him to MJ down the line. If she came back, she'd likely already be out of the way and wouldn't try to take back Peter out of respect as she's a 'shipper-on-deck