I wish I knew the answer certainly, but my guess is more than average. I'm a former military brat and I grew up witnessing extramarital free-fall (men and women). It was so prevalent in every duty station I lived that I thought it was normal. It wasn't until I became an adult and was away from the military lifestyle that I saw how much more it existed there than in any other setting I've lived in. Oh, and the hyprocrisy that existed among the people doing it was just plain shameful; everyone was doing it but they were all quick to rat each other out as though they themselves were innocent. And yes, the military supposedly frowns on such behavior, but since everyone is doing it, it is more acceptable now than ever before. There is no longer harsh punishment for being found out. It's just one big orgy.
I am not aware of any statistics that document this. Under the UCMJ (Universal Code of Military Justice) unsolicited carnal knowledge is a crime. This means that anyone asked the question is likely to answer it negatively.
Fighter pilots in general are egomaniacs. Feeding a large ego often (not always) involves many romantic conquests. Often the allure of such a high-status job attracts young ladies - a dangerous combination.
Promiscuity is a part of human nature. There is a saying, "women need a reason to have sex, men just need a place". The desires and needs behind the sexual act may differ between sexes, but the physical need is the underlying, motivating force. Mental, egotistical, emotional, social or other concerns are peripheral to the physicality of the need. Pressures in military life may seem an adequate excuse, but as I said, sex is natural. There are immense pressures fighting the peace as well. To me, if someone gets drunk, acts a fool and does/says something they regret later, then applies the excuse "I was drunk", well that just doesn't cut it to me. Perhaps you should have laid off the booze. The same applies to marital infidelity in my books. You took the steps, said the vows, declared your undying love for the other person, then a little time and distance separates you and suddenly that all goes out the window? This is the sign of a weak intellect, an poorly defined psyche. Clearly the person was talking out of the side of their mouth when they were blathering about eternal love and need to lay off the daytime soap operas and develop their own identity. All of the systems we apply(mostly to others whenever we can), including the marriage institution, for that is what it is, an institution, are all completely un-natural. Antithesis to the human condition, anathema. Personally, I do not believe we are the most evolved species on this planet, we are simply the apex predator, the most complicated animal. We can't seem to complicate or lives enough because it becomes a bigger mess daily. Considering that humans are at odds with everything natural around them, especially themselves and others(we do not adapt to our surroundings, we adapt our surroundings to suit us, hence, still evolving, not "evolved", whereas a lion or tiger or bird or elephant integrates into the existing environment - perhaps the "dumb animals" are actually of a higher evolved state than we are), it should be no surprise to anyone that when the monkey is taken out of the cage, it's desires flood to the surface and off it goes, running wild. If your spouse has been philandering, get a lawyer and get a divorce, it is that simple. In Roman times, when a man's wife was cheating, he was well within his rights to kill her and the man she was cheating with as well, she being his "property". Clearly, we do not need to return to a Roman stance of conducting ourselves, (although some in society greatly desire exactly that) for the Romans were at least as brutal as they were civilized. Affirm YOURSELF, you do not need another person, a significant other to affirm who you are, nor any parent, friend, organization or comittee of thousands. You affirm yourself, then go out and find someone whom has also self-affirmed and then you compliment each other.
My husband and I are both in the Army. I have been in 3 years longer than my husband. The numbers that have been told to us are that in regular jobs in the Army, the divorce rate is about 60% and in high risk upwards of 75%. I have known this to be quite true, as much as I hate to admit it. It may not be true everywhere though. Most of my friends have been divorced or cheat on their partners. Although my husband has never cheated, I have caught him and am now trying to work through him going on internet dating sites and pornographic material to cope with me being pregnant and having a different body since having my baby 6 months ago. Most of his buddies in his unit are single and so I believe that he tries to get them to like him by acting like them. He and I grew up in the same town, where he and I were both not popular. I didn't seek to be in the "in-crowd" because I was happy where I was whereas he was constantly trying to fit in because he wasn't the smartest kid and he told me that he felt the need to fit in.
To answer your question, the military very highly discourages cheating behaviour and reprimands anyone caught doing so (not in all cases though, sadly). As Boris mentioned, there are UCMJ codes that can be used against a soldier cheating on his or her spouse.
I am also not aware of published stats on this, although individual counseling is a large part of what I do. So I will provide my perspective only from that experience.
Overall, I have found there is a mindset that can occur within the military environment that we might consider. 1) Men, in general, are historically more promiscuous than women are in western and other cultures, largely due to socialization, and somewhat due to biological (testosterone) factors. Many clients have shared with me that they are often on assignment for long poeriods overseas or away from family, and they see promiscuity not as a betrayal of their values, but as a result of long periods away from families where they find that the behavior meets their primarily immediate, physical needs. In other words, most of these men have shared that in general, they feel promiscuity on their part is not a statement of disloyalty of or betrayal of love for their wives or families, but as a sexual acitvity apart from this, whereas their wives (obviously) see it differently. I don't make judgment on this, but only offer it as my own experience as counselor and confidant. (as a woman, I obviously don't see it similarly, but this is not a part of your question.)
We know the military is still overwhelmingly comprised of males, and this socialization and attitude among males (acceptance of promiscuity among males in our culture) is likely more acceptable than in overall society because of gender behavior pattern reinforcement from other males.
2) On the other hand, many individuals who have been drawn to the military life and values have and come from conservative family and other traditional values themselves-- family, "patriotism", loyalty, religious values, etc. There are, in western culture, many individuals who strive to live according to these values, and hold them (and their wives and families) precious--and honor that in their actions even in the most difficult times.
3) People are susceptible when their self image is overly reliant upon the attention and acceptance of others. Many individuals are drawn to military service for the excitement and proof of value the career can provide. I agree with Boris in that fighter pilots and poilots in general are perceived in some cultures to have a superficially perceived status based on their professional role. (Part patriarchal socialization, and --unbelievably-- part primitive/instinctual attraction of females toward the nstinct of seeking "strength" and "protection" to perpetuate the species!!) Kind of mind-blowing in this century, but the notions are residual and slowly resolved.
In light of these considerations, my experience is that infidelity is slightly higher among military personnel than not, though the difference is hardly significant.
I hope this helps answer your question. Please let me know if I can provide further info that might be helpful to you. My personal position is that fidelity is a higly personal matter with individuals, and to generalize (to military or other segments of population) is generally invalid and not wise when considering personal circumstances.
Thanks to all of you who have written in. My husband admitted to a one time affair. I forgave him but I still have these nagging fears. He said he loved me and it was a mistake. I will never know for sure since he died of cancer. I just wish I could move on.