It seems to me that the concept of restraint in the acting performances in BBM reflects not only the general character of the society in which the story is set... but it's an essential factor in the dynamics between Jack and Ennis that allows the relationship to move forward.
If too many truths were uttered and aspects of their relationship were thoroughly discussed or surfaced to the fullest... it seems clear that Ennis especially would be completely spooked.
Taking an example from early in the film (rather than the lake argument)... the "I'm not queer" discussion seems to be entirely about having the mose spare and restrained conversation possible about the huge thing that's just happened between Jack and Ennis. There are a million things that could be talked about between them here... they could confide in each other on an honest level about how they're feeling about their sexuality... they could even confide in each other simply about the level of confusion that must exist for each one on the question of sexuality at that point. Instead, they cover almost everything over with a lot of silence and one enigmatic agreement about the "one shot deal" (does anyone really understand what that means... even Ennis?) and another enigmatic/ fib-like agreement about "I'm not queer." But, this exchange seems to lessen the tension between the two. They seem content to just be close to one another here. I feel like when Jack taps his toes together (very, very subtly) it's almost like a signal that some level of tension has been let go (at least for now). And, I think the fact that TS2 follows from this... and that TS2 (which is clearly one of the most overtly intimate scenes) occurs in nearly complete silence (aside from Jack's whispered "s'alright" and the controversial "I'm sorry") between the two is critical for this interaction to be possible between Jack and Ennis. Again, I think if too much openness or discussion happened here... Ennis in particular would be spooked and scared off.
Throughout the relationship Jack seems to measure his words and what he reveals to Ennis very carefully. I think Jack's restraint is very much forced and deliberate. Perhaps he learned a lesson of sorts during the "prayer of thanks" camping trip... where he learned how painful expressing too much could become. Asking Ennis to move in with him and start the cow and calf operation (all in sort of a nervous rush... the way he actually makes this proposal verbally) was a big risk and a moment where he's exposing more of his desires and thoughts on the relationship and their future than in many other instances. The rejection of this offer and the spare terms for the relationship spelled out by Ennis may almost feel like a punitive reminder of the risks associated with saying, suggesting or offering too much.