If a person has any amount of self-esteem and love of self they will always have boundaries and "deal breakers". In other words they will not tolerate certain treatment in a relationship or marriage and therefore there is no "unconditional love". By that I mean this person would choose to walk away from a toxic relationship as oppose to staying for the sake of staying.
Each of us is entitled to have our own "deal breakers" which might include cheating, verbal abuse, physical abuse, displays of violent temper, overly jealous and possessive behavior, alcoholic/drug addiction, abusing children, emotional and sexual neglect...etc Essentially healthy people do not choose to stay in relationships that cause them extreme unhappiness.
With regard to cheating both men and women have been known to cheat on those they love. The primary goal of any cheater is to hold onto all that is good in their primary relationship while addressing their other "needs" on the side. Cheaters are not looking to go through a messy breakup or divorce nor do they want to replace one relationship with another.
Hypothetically assume a man is sexually frustrated in his marriage. The vast majority of men are not going to run down to the courthouse to file for a divorce! They're not going to choose to move out of the house into an apartment, become a weekend dad, pay child support, and possibly alimony simply because of sexual incompatibility. It's not a "deal breaker" for them.
People cheat because they want to "have it all". This may entail the need for variety, an emotional connection, or a feeling of being desired and appreciated. Most cheaters do not expect to be caught. This explains why they're willing to take the risk.
Having said that (why) is unimportant for the most part because no one really cares about why. Asking why is a rhetorical or reflex question. In the eyes of the betrayed person there is no justification. No answer would satisfy them.
Instead of trying to find a way to have "unconditional love" or offering "instant forgiveness" it's probably best for the betrayed person to take a few weeks alone to decide if cheating is a "deal breaker". People who forgive immediately usually do so because dealing with the combination of cheating and a breakup is too much for them to bear.