Story structure question: Brokeback first tent scene

So, to folks who have seen it, there's a gender split argument in our screenwriting group about the (lack of) motivation for Ledger's actions in the first tent scene. Two of us chicks didn't feel the Ledger character "needed" to make the choice he did, thus weakening the power of that scene (when I saw it, there was a lot of giggling). But the guys in our group felt that the conversations between Ledger and Gyllenhal's characters made it perfectly clear that Ledger's character had to respond to Gyll's offer in the way he did.
So did we miss something because we are chicks? Was his behavior as motivated as, say, Bruce Willis' character in Die Hard when he initially chooses to save his wife the way he does? As motivated as the early choices made by Hugh Grant's character in 4W&aF? etc.

Interested in what folks think, as it was for me the only weak link in the story.
Thanks!
Mysti