It's been obvious that Josh and David have been bonding over the past couple of months and it's been obvious that Josh has been attracted to David even as all of the program's eligible women have been throwing themselves at him. Apparently, Josh made the decision to make his move on David during the previous episode.
Shona, David's girlfriend, made a joke about David being jealous of Josh and how he can't keep Josh all to himself. Josh overheard this comment and decided that he was going to arrange a date night for the two men. And that's how Josh apparently saw that evening, as a opportunity for the guys to hook up.
Not that he gave David the opportunity to make that choice on his own. Josh made sure that the guys had sex with each other. He got the GHB. He arranged for them to be alone. He made sure that David drank too much and took a large dose of the drug. And he got David all emotionally worked up.
It's unclear if Josh has roofied anyone before David -- but he's definitely thought things out pretty well so it wouldn't surprise me if he's done this before.
Incidentally, I saw some advanced images from this week's episodes. It's pretty clear that David knows that Josh raped him and that he doesn't know how to move forward because of embarrassment and shame. He will transfer his anger onto Shona and end up running off to see his father -- presumably with plans to emigrate with his father to New Zealand (some recent development that we learned of early last week).