I've asked my ex military partner this question and they said it is simply because these guys think the fat ones have low enough self esteem to tolerate being cheated on while on deployment but they're too insecure to cheat back. This is apparently only discussed amongst the boys, "Get you a fatty, she will just sit at home and raise your kids while you can still see all of the beautiful women in the world. You"ll barely have to see her". Unfortunately with all of the inflated egos of pig culture this will probably not be effective anymore.