How likely is it for a boyfriend to cheat while he is in the Marines?

Little to zero. The Marines are very hard core. Even if there was a girl he was head over heals for in his unit, there's no way he'd be able to. He'll be working hard for his country, don't distrust him. Give him some credit, he's going to go through hell to even make it to the Marines.

no chance they are surrounded by guys and the very few girls there are even if they are pretty will be gross and disgusting all the time

aren't they mostly dudes? unless he turns gay... O.o i dont think it's very likely lol