You may be exactly right, Derby.
Another factor that may contribute is the fact that emotional and physical needs don't get placed on hold when a service member is away, and polyamory is an ethically valid approach to addressing those needs in an honest and open way.
Polyamory beats infidelity any day.
Begin short hi-jack: I would like to take this opportunity to thank the members of the American military and our allies for their service. I know I live freely and joyful because rough men and women stand ready to do violence in defense of our liberty. Thanks are not enough, but that's what I have to offer. End of hijack.