This is just SOOOOOOOO Dustin&Mark!! and I really LOVE it!!!
The thing is. Dustin is kind of too busy to get laid. Because he's pretty much constantly working on Facebook, or going to events because of Facebook, or going to meetings or trips because of Facebook, and for Mark it's like that times a thousand plus even more press shit.
So Dustin doesn't really have the time to go get laid. Not that the Facebook employees aren't fairly hot, but Chris would frown at him and really, that could just get awkward.
So Dustin comes up with ingenious solution to his problem: clearly, he should just have sex with Mark! Because Mark is also too Facebook busy to get laid, and they're already best friends and have done much more potentially humiliating and/or uncomfortable things together! So it would totally work out great, and it's not like Mark's totally unattractive, so there's that too.
And Mark is kind of dubious about the whole thing but naturally Dustin wears him down and it's kind of great and really, really hot and generally awesome and they're still best friends and it's not awkward. Except, as time goes on, Dustin can't help but notice there are people who flirt with Mark (and okay, yes, there are people who flirt with Dustin sometimes, but he'd noticed that before but he definitely did not notice the sheer number of people who hit on Mark) and maybe he's kind of not okay with that. Because Mark is his friend and Dustin's looking out for him and also maybe he kind of wants Mark not to want to kiss people other than Dustin and to adopt kittens and create amazing things and wake up every morning he can with Mark.
Luckily, when he finally awkwardly confesses this, Mark just sort of rolls his eyes and is like, you're really stupid. Obviously we're going to do that.