they don't really love them. they may think they do, but if you love someone you would not justify betraying them and causing them pain.
i do not think its the other partner's fault if they cheated. maybe that other person is just a selfish f***ed up person. if you are unsatisfied with the relationship tell your partner and try to work it out..or end the relationship since your clearly unhappy, don't f*** someone else and then try to say "Oh I was unhappy" TOTAL COPOUT