It is simply not true imo that a child needs both parents to develop. Look the the divorce rates today and then look at the kids. There is next to no correlation between kids with issues and divorced parents.
Divorce isn't selfish given that it's only usually done when it is believed that, in the long term, both parents and any children that they will benefit from it. As has been said before in this thread, better be brought up by one parent than two arguing (or worse) parents, right?
Although you say it in the marriage vows and stuff ("'til death do us part") it's impossible to predict the future or know for sure that you made the right choice with your partner. Divorce is not unusual or inexplicable, and not at all selfish with everyone's best interests in mind. Hell even if the two parents were unhappy but not the child I still don't think it's selfish; two unhappy people and one happy person could be turned into two happy people and one temporarily unhappy person who will hopefully see why it was for the best one day.