05/31/2011 07:21 pm ET Updated Jul 31, 2011

Congratulations Kate and William! Did you Sign a Sexuality Prenup?

After an estimated 22.7 milliion viewers watched the wedding of Kate Middleton and Prince William last month, I wonder if any of them thought about whether this marriage too, like Charles' and Diana's, would end in infidelity and divorce?

Gazing at Kate's calm, cool, and collected demeanor, pundits around the country described her composure as coming from the fact that she is a mature woman, unlike Diana, who was only 19 years old at the time of marriage. These same pundits offer up the fact that the couple have had time to get to know one another, even live together for the past several months on the island of Anglesey. One wonders if she is as innocent and noble as other English heroines at the beginning of their romantic relationships like Jane Eyre who believes completely in her own and her lover's ability to stay true if you are honestly in love. But does marrying when one is older and living together before marriage and belief in true love act as a barrier to infidelity? I don't believe so.

As a couples therapist and sex therapist who sees couples before, during and after an affair, I can say that in my experience what most couples suffer from is a lack of transparency when they decide to be a monogomous couple? What do I mean by transparency? Just as most couples discuss where they want to go for dinner, which invitations they will accept, when and where they will marry, couples should discuss what each person considers cheating or infidelity. But couples don't do this for the same reason that couples refuse to create and sign a prenup; they don't want to come across as distrusting to their beloved. Although The American Academy of Matrimonial Lawyers says about three-quarters of its members have seen an increase in prenups in the past five years, I can bet my hat there is not a word in these agreements regarding what constitutes infidelity.

The reason I think couples need to talk clearly and in much detail about this subject is that unless a boundary is articulated there is too much room for interpretation. For example, a couple comes into my office because the wife is extremely upset that her husband is using pornography. Although she was aware that he used porn to masturbate before they got married, she expected him to stop using it after the wedding. Why? Because she regards it as cheating. For the husband's part, he thinks that porn and masturbation is a natural part of his sexual life and feels frustrated that his private sexual life of masturbation should be governed or controlled and judged as disgusting by his wife. Had they been able to discuss, debate, and decide about this issue before the wedding it would have saved both of them much angst and pain.

What about if a woman is flirting at a bar with a man who isn't her husband? Her husband watches this and becomes jealous and angry because he regards this kind of behavior as cheating. I'm not clear if Charles was having phone sex with Camilla at the start of his marriage to Diana but I'm sure if Diana knew this at the beginning of their courtship the wedding might have been cancelled. And did our own Hilary and Bill at some point redraft their sexual agreement? After she discovered his sexual relationships with other women when he was still Governor of Arkansas, did Hilary sit down with Bill and draw up a "Don't Ask Don't Tell" policy regarding sexual relations outside the marriage? Or did she tell him this better not happen again, knowing in her heart that he just couldn't follow such an agreement and being dishonest with herself?

We will never know the whole truth. However, couples today would do well to articulate clearly and in much detail what kinds of activities they would consider infidelity. It could be the most challenging discussion they would have but might save them much heartache and many dollars paid to attorneys and therapists later on. Right now Kate and Will are in love and the world is riding on that fairy tale because it offers them hope, renewal and belief in ever-lasting happiness. But if they haven't discussed the temptations that are out there to every monogamous couple they haven't affair-proofed their marriage from where I stand. As much as I love a good show and the wedding was full of pomp, circumstance and beauty, the real world awaits them back on on the island of Anglesey and the expectations of them as the heirs to the throne.