Unfortunately, forgiving is the easy part. Forgeting is what will make it hard. Many relationships can move past this and continue on. The only way it can be forgiven is when the persons that were hurt agree to forgive... It can't be forced or you risk the chance of pushing them away for ever. No matter how bad or how small the act seems to the person commiting the act of infidelity.. The victums and I do mean victum(s) will see it as a betrayal of trust and of thier love... These pains run deep and never truely heal completely... The only chance a relationship has after this kind of pain is inflicted is COMMUNICATION. The key to making any relationship work is to talk about your feelings and problems with each other, and if you both truely love each other... "This to shall pass". It is ok to feel hurt, you are only human after all, but humans have a built in since of compassion too.
I hope this was helpful, and that I have answered your question completely.
Your Friend in Health and Happiness,