I think almost anyone in a long-term relationship has this desire, and it has nothing to do with the strength of the relationship. Men and women are not born to be monogamous. Monogamy is almost unique to Judaism and the religions that it spawned, and it came about by trying to control women's sex lives in order to control the line of inheritance from father to son. If you don't control a woman's partners, than any man can be the father of her children, and men become, in the scheme of things, largely irrelevant.
The majority of cultures had (and many still have) non-monogamous marriages, recognising the reality of our desire to have sex with other people.
As to the West, we haven't been successful, and the percentages are as high as 30% to 40% unfaithfulness. And time and again, the famous people who get caught out are often happy together but still have a desire to stray. Those who aren't in the public eye are often happy together, but still have a desire to stray.
I agree with LadyShady's view that one-off sex is not the end of a relationship, but being romantic with another person may require some deep thinking. I have been in the latter, and my experience was that I could love one person and love another person, and my love for the first wasn't affected by my love for the second. Love is not either / or, it's not equivalent to possession, instead it's additive.