Do you mean the conversation they supposedly had, that we don't see?
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About either / or: I find your statement interesting. I see Ennis as a straight man who fell in love with Jack; and in fact, Heath (and maybe Jake) said that in interviews. But that theory was shouted down in discussion forums as preposterous; perhaps it had to do with a gay agenda. Proponents of that stance were taking Brokeback and running with it: finally a mainstream movie showing a gay relationship. Ennis was gay. Full stop. As a straight woman, it might be that I'm romanticizing their relationship. I see Ennis & Marc as straight guys who fell in love with one man. As for their future, Ennis (IMO) never had another relationship with a man. Marc's future is still open to it, but (again, the straight female romantic) I want him to be with Kay.
But we do. Right after Marc flees the club, he drives home emotionally a wreck to tears, uttering "Sh1t, sh1T...", next we cut to Marc approaching Bettina from behind while she's sitting on the deck, no doubt playing this in her mind. Their conversation says it all. Marc, "I cannot go on like this." Bettina, "Me neither." long pause, "Me neither" It is very subtle but if you watch the camera focus it shifts: First on Marc with the "cannot go on.." then to Bettina with "Me neither..." and we get a really good look her realization and despair knowing she's lost Marc. Then the camera shifts back and refocuses on Marc before cutting to him entering the academy training room again. Hmm, where did we see this before? The "I wish I knew how scene in BBM?"
That's the conversation, both spoken, unspoken and the lack of any intimate eye contact - says what I'm leaning on.
Re: I agree with you and I remember those heated debates too well. Once one accepts sexuality is or can be fluid, it is quite possible for a "straight" guy to fall in love with another man. It's when we box ourselves into this either/or that the "must be's" surface. In both cases, with Marc, we are clearly shown that another guy is no substitute for Kay and with Ennis we learn from what he said and did there were no other's besides Jack. V.