You know, it's been a long time since I read Alias so I could be wrong here but I'm pretty sure she wasn't unfaithful. Here's what I remember:
1. Luke and Jessica have a one-night stand; she goes back to him to talk about it but at that point the chemistry fizzles. 2. After a brief period, she starts dating Ant-Man, not especially seriously. 3. She discovers she's pregnant with Luke's kid, and breaks it off with Scott. 4. She tells Luke, with whom she still has chemistry, and they decide to try for a real relationship and raise the kid.
So yes, rushed, but Jessica has zero history of infidelity.