So in other words Marines do often come from at least lower-middle-class backgrounds. Which perfectly explains their relative hardheadedness on this issue versus branches which recruit from higher socioeconomic classes like, say, the Air Force. That explains it, then. Their conservatism on this issue is a natural extension of the basic conservatism of the low classes.
I dont see where it says that. I think you are reaching somewhat with this class thing. The Marine Corps is the youngest and by far most male service. It is also the only service that has separate recruit training for men and women. It is structured foremost as a combat, infantry branch. As a young service, with less officers and a well ingrained macho tradition- you will invariably get some anti-homosexual undercurrents- as you would in most football locker rooms.