Interesting thread! A question I would like to put to all those who are conflicted by their feelings about "cheating" on their spouses (guilt or otherwise) - In your relationship, who is really cheating who? In marriages, I have never fully understood why one spouse wants to deny the other spouse a fulfilling and richly rewarding life and place so much emphasis on sexual fidelity as a key factor to defining what a rich and rewarding life is. Many men, and women, have a basic desire to explore who they are, and want to experience as much of life's experiences as they can. Taking a lifelong sexual vow, and being forced to keep it, either through self-restraint or from the threat of a breakup of a relationship is both naive and unrealistic. Couples should work together to see that both partners have the most rich and rewarding lives possible, and to deny the other such an opportunity is, IMHO cheating.
As Viper73 said, sex is sex and it does not define a relationship. It is only one aspect of a relationship and there are other far more important considerations such as being supportive, working together as a team etc. If one wants to have a sexual experience(s) that a spouse cannot provide, what is the real harm in going out and having such an experience? I submit that by fulfilling such desires one is actually doing their spouse a favor since it will not lead to feelings of resentment or unfulfillment of one's goals in life. As long as one does not put their overall relationship in jeopardy, then no harm has been done and in fact their relationship is likely to be improved by such experiences. So again, who is harming who and who is cheating the other? Why feel guilty about something you desire and act on that is as harmless as having a purely sexual experience with someone of your same sex?