No, we do things on a regular basis that cause pain to others. I don't believe it cruel unless the intention is to inflict pain. People who cheat don't INTEND to inflict pain.
I did my best to stay away from this discussion because is one of those topics where people never acchive nothing or come to a clonclusion, but as I am seeing that you are trying to influence people with your views I really could not stop myself from intruding (yeah weak I know)
Plain and simple you are wrong, most of the cruelest acts done in humanity where not done with the intention to cause harm they were selfish acts for the satisfaction or benefit of those who did them. prima notte or Droit de cuissage:
it was not intention of the feudal lord to harm the young brides or to hurth tha stbh, they were just taking adventage of the laws of their times stipulated by greddy kings to placate their feudal lords for the high taxes imposed. pedophilia in old empires:
it was and honor for the kids to be selected as the consort of emperors and kings and the parents were well remunerated during those times. Old treataments for metally ill people:
lobotomies, electroshock therapy, cold water treatment, the intention was not to hurt the patient but to heal him and doctors at that time honestly believed that they were benefical for patients (and some in fact have positive results but as side effect of the damage caused more an illusion than a reality). working kids during the industrial period:
now days a kid under 15 being forced to work in unthinkable back them it was normal to expect the kids to contribute to the family income and horrible jobs as chimney sweep were created for them, which later proved to be mortal.
I can go for hours with examples as these where the perpetrators where not consciously harming the affected part and still we call them inhuman and the cruelest now days, real life persons are not like movies where the perpetrators are evil villians who rejoice in the pain of others and they are evil 100% of the time, if you have seem threads of marital predators who go specially after married women some of them even have a deluded mentallity and think that they are making a favor the the husbands of the wives they are bedding, they describe how the wife was about to divorce but thanks of them now the wife is happy and satisfied reamain at home, some even think that they saved the marriages, they don't think in harming the husband or destroying him to push him to commit suicide they are just satisfaying their selfinesh.
finally if you still don't think that infidelity is cruel, let's review the definition of cruelty by the Oxford dictionary in the law section:
"Behaviour which causes physical or mental harm to another, especially a spouse, whether intentionally or not
in the case you mention infidelity is cruel precisely because the perpetrator didn't think in his/her partner during such a destructive act, what could be more cruel for the affected one that to know that something that is so hurtful and destructive to him/her was done for his/her loved one with even considering him/her before shattering his/her whole world?.