I'm glad you didn't make Edward marry Heidi.  In my culture, back in the days, and still practiced in our country of origin, the family will make the wife of the late older brother marry his younger brother, especially if there were children (sons in particular) or wealth involved as to keep it all in the family.  It was a selfish thing to do.  And honestly, I think that as adults, we go into something fully aware of our actions and with every one of our decisions come consequences.  So if we're willing to take actions, we should also be willing to accept the consequences, whether good or bad.

Now, I understand that Bella can be confused and angry after Edward's revelation.  But she also needs to accept the fact that they weren't together during the time when it occured.  Yes, it is very hard to accept.  But I think men and women differ greatly in how we manage our emotions.  As women, if we feel down, the last thing we want is sex.  For men, I'm incline to believe that when they feel down, sex is a relief.  I'm not saying what he did was right.  All I'm saying is that we look at it from both their point of view.  He didn't necessarily cheat.  She dated too even if she didn't sleep with anyone else.  They both tried to move on.  Forgivesness is probably one of the hardest thing to do because to truly forgive would also be to accept that person's faults along with his goods and to forget about the mistake.