Thought this had comic potential. If you like reading gay sex stories you can see https://www.reddit.com/r/gaystoriesgonewild/comments/7hu7c2/hook_up_late_night_barracks_mixup/ but

TL;DR male Marine finds another Marine to give a blowjob to on GRINDR. He goes to the barracks and room number he got, and sees two male marines in bed, one kinda curled up and maybe sleeping, the other spread out with a raging hard-on. So he goes and gives a blowjob to the guy with the hard-on. His phone starts going crazy with messages so he turns it off. When he finally leaves he turns his phone back on and sees all kinds of messages along the line of "dude that's my roommate, what the fuck" then "fuck I don't believe he's into this". So he goes back and blows the right guy.