Infidelity in a relationship be treated as a dealbreaker because it tells all about the lack of character of the person who did the cheating. You shouldn't make the promise and commitment to be monogamous and faithful to someone if you aren't going to keep it. Once infidelity happens in a relationship, the trust is gone.
Infidelity in a relationship is an absolute deal breaker in my opinion and if I were aware of it, that would be the end of the relationship. Some people have different views on this or try different paths. Their decisions are not inherently bad, but it shouldn't be assumed that infidelity is the common or accepted practice.
Everybody makes mistakes. It is human nature to lust after others. It is biologically determined. In most cases of infidelity, I think, the act could be grounds for ending a relationship. This, I think, isn't true for all relationships. Some relationships are of a quality that one would think twice about ending them, some people are worth a little forgiveness.
I think it is up to the individual whether infidelity in a relationship is a deal breaker or not. Some are able to get over the act of cheating by their mate. Some relationships are open to cheating, so the define infidelity as a deal breaker would for a relationship will be a case by case basis.