Relationship psychologist Allison Keating from Dublin's bWell clinic says even if members are only exchanging flirtatious emails online, they are damaging their relationships.
"Going on to these websites is cheating," she says.
"Core values and bonds of a relationship are based on mutual respect, trust and intimacy, and by having a 'harmless flirt' with a stranger you're breaking those."
She adds: "If you need this extra stimulation, rather than look for it cybernetically, why not look to your partner?"
While many extra-marital affair devotees assert that the secret trysts make them better wives and husbands, Allison is unconvinced.
She says: "I have yet to work with one marriage that has been made better by dealing with the tsunami of destruction that an affair creates."
At Christmas many people can look at other people's relationships and think they look perfect compared to their own, but all marriages have their problems behind closed doors. Before signing up for an affair, Allison's advice is to sit down and write out what's lacking in the relationship.
If it is fun, spontaneity and excitement that's missing, then think back to a time when you had it in your relationship and figure out a way to bring it back.
She says: "It's easy to create a spark with someone you have no emotional connection with, but it is the process of developing your relationship that will stand the test of time.
"Always remember the consequences of your actions, as sometimes the short-lived and hedonistic do not provide long- term happiness that a nourishing relationship can."
She adds: "If you feel completely emotionally and physically disjointed from your relationship, talk with your partner and if you need further help seek professional help from an experienced relationship psychologist."