If someone is going to cheat, they'll cheat whether it's with an old flame they found on facebook, an old flame they saw in the grocery store, or some cute server bringing them drinks at a guys/girls night out.
I think part of the problem today is that people just don't consider marriage as a forever commitment. I made a promise to love and stay with my husband until one of us dies. We take it seriously. We both are of the opinion that divorce is not an option, and if there are problems we will have to work it out. Just having that mindset from the beginning has kind of shaped our marriage (in the right way, IMO).