Generally, online affairs are easier to perform and put the agent in a less vulnerable position, as the chances of getting caught or being hurt in other ways are considerably reduced.
People, consciously or not, consider their online sexual relationships as real—they experience psychological states similar to those typically elicited by offline relationships.
Whereas people having online affairs tend to understate their problematic nature, their offline partners typically do not see difference between online and offline affairs: A lack of direct physical contact and face-to-face meetings does not diminish the sense of a violation of their vow of exclusivity.
The fact that most of these affairs are concealed from offline spouses is indicative of the possible harm.
Nevertheless, since online affairs are real they do often cause actual harm to one's primary, offline romantic relationship.
Accordingly, many people will be just as disturbed about a partner's online sexual affairs as they would be if they discovered that their spouse was exchanging steamy love letters with someone else.
It's like reading an erotic story and masturbating to it.