Earlier quoted context omitted.
Infidelity is certainly harmful to someone (emotional harm is a real thing, not to be discounted) but so is exposing people's private lives and peccadilloes. Infidelity is something that should be resolved in private by the parties involved. Whatever you do, someone will find it offensive or harmful to them, for some definition of "harmful" -- even the way you dress, or the music you listen to, or the books you read,…
Considering infidelity can lead to non-consensualy exposing people to STDs and possibly lead to rape (depending upon how you define consent).
Exposing people to STDs is a problem which can also happen regardless of infidelity. Besides, are you saying if you practice safe sex while cheating then it's morally ok? Is it simply an issue of safe sex?