This jealousy you speak of is but one of many emotions we married guys sometimes experience. Its usually all wrapped up in a hugh adrenaline power ball ... anger, jealousy, excitement, regret, envy, desire, etc. and its HOW we, each, react to that power ball that is important. Some guys find it sexually stimulating and get erections, others handle it more negatively.
Once a couple has been together for several years, it often takes something extra to stimulate and maintain the sexual excitement as it was originally. And for some, inviting another person into the love crib is that stimulus. As you said, as long as it works for everyone, that's good for the relationship. A marriage, however, is a special relationship between 2 individuals, and its important that the 3rd person respect that marriage and only add positives to it.
