The OP may be straight of mind, but bi of body. Or even gay. Who knows... But as long as he chooses to approach himself as "straight", it will be so, however in-denial.
OP: in my experience, establishing masturbation as a comfortable topic of conversation is a good way to segue into a hook-up. And it's a topic many straight guys are totally willing to go with. ESPECIALLY with their BFFs. I've used this as a way into straight seduction many-a-time
Make a joke one day.
YOU: "I'm gonna go."
FRIEND: "What are you doing?"
YOU: "Gonna go do my thing."
FRIEND: "?"
YOU: *shyly jerk off your invisible cock*
Your friend may react oddly, but I find that guys usually think it's hilarious. Most straight guys probably feel ashamed for wanking to begin with, so your bringing it up sort of "relieves tension"

. Now, the big hope with this tactic is that the next time HE's leaving you to masturbate, maybe he'll be like "I'm going to do my thing" -- Congrats! now you guys are talking about masturbating. HOT.
Eventually you want to get to the point where you can say something like...
YOU: "I really want to do my thing, but my roommate's in our room."
FRIEND: "!!"
YOU: Dude, we should look at porn.
OK, now the dicks come out. Who knows where it goes from here...
Have fun. I can't promise that this will work for you. I doubt it will, necessarily... lol... but it HAS worked for me! Seriously.
Disclaimer: Not all straight guys are the same... but there are a lot of really similar ones lol.