I argue in 2 Books About Singlehood that loneliness is not a condition reserved for singles. Even people who have a special someone can feel it. Brokeback Mountain proves this point.
I am going to set aside the issue of adultery (which no, I don't think being gay justifies) since adultery is a condition of the movie, not the point. (Jack claims he is willing to start a new life; Ennis is unwilling to sacrifice what he perceives as obligations. The writers make no judgment either way.)
The point of the movie resides in the men's weeks apart, in Ennis's pained fury when they are called back from Brokeback a month early. It lays behind Jack's line towards the end: "Tell you what... the truth is... sometimes I miss you so much I can hardly stand it." It culminates in the gentle kindliness of Jack's mother: possibly the only person in the film to "get" Ennis and to accept Ennis' role in Jack's life without reservation (she treats him like a mourning spouse). It resides to a large extent in the men's weeks together since the weeks together reinforce what they lack apart.
In a much less uncertain and far happier vein, yaoi tackles the same issue: chance meetings, chance happiness as a non-guaranteed (and incomplete) answer to loneliness or alienation. Its prevalence explains why the theme--in a movie like Brokeback Mountain--transcends the subject matter, speaking to a wide audience.
I personally believe that heterosexual paperback romances can be enormously insightful re: romance, relationships, men & women. Yet at the back of them--as at the back of most discussions in our culture--is the idea that everyone is entitled to such an arrangement. Even in cultures with a growing number of singles--in sum, the industrial world--the default of "every man will get a woman and every women will get a man" is so strong that trying to convince people that (1) relationships aren't made to order; (2) being single can be choice; (3) being single can be accepted rather than railed against = hitting the proverbial brick wall.
Single person meet society's brick wall.
Consequently, a single heterosexual woman (who loves romance) can begin to feel somewhat of an outsider despite belonging to a culture that ostensibly would support any of her romantic entanglements. Unlike a gay man (or lesbian woman), I have no stigma to battle . . . other than the pervasive stigma that I am an anomaly in my culture, a factor "gone wrong," because I haven't complied or haven't behaved as if I wish to comply with the expected outcome. (As Elizabeth discovers with her rejection of Mr. Collins, failing to comply once the object has been produced, no matter how superficial in essence, can cause great consternation by others.)
Movies like Brokeback Mountain, books like Maurice, and the entire subgenre of manga yaoi romance allow for the reality of loneliness/alienation, even if some of these works resolve it more smoothly than Brokeback. Loneliness, alienation, and confusion (which confusion doesn't preclude the existence of a significant other) underlies the literature in force, helped--in Japanese manga--by the concept of mono no aware, the idea that one must grab onto beautiful or heartbreaking or quiet moments as they come.
Such reminders can be painful but also solacing. It isn't weird to feel that loss. It isn't odd to keep going anyway. It isn't strange to negotiate with it--because sometimes the loss is worth the benefits.
It's human and real.