It’s not unlikely. After all, John Winchester was a Vietnam vet who grew up in a small Middle American town; that kind of background isn’t traditionally the most conducive for fostering tolerance and open minds. And we can see how uncomfortable being the target of Dean’s interest makes John, especially during the Impala scene. (Although I think John’s more weirded out by this strange man’s behavior than by the interest itself, his body language still screams “no homo”.)
When we first meet Dean, he’s obsessed with proving his masculinity (which in part he attempts to do by as having as many casual affairs with as many women as possible). A need like that, it doesn’t grow out of nowhere. It evolves when a boy is made to feel less than adequate for being who he is, by the man he inherently admires most: his father.
We’ll never really know what passed between Dean and his father; the only account we have – John’s Journal – is fringe canon at best, and inherently biased towards John’s perspective. But if his son were to exhibit signs of sexual interest in men, I think it’s fairly consistent with what we know about John Winchester’s character for him to have tried to squash it as best he could. In the world that John Winchester originated from, men who like men aren’t manly. They are the opposite of manly. They are weak, and sinful, and any loving father whose son displayed such tendencies would do the right thing and attempt to rid him of them as best he could.
Although obviously signs exist of Dean and Cas’s “profound bond” in Seasons 4 and 5, I don’t think it really starts to take on specifically romantic overtones until Season 6 – which, not coincidentally, is after Dean has matured enough to set aside his father’s shadow and become his own man. “Swan Song” was a symbolic rite of passage, wherein both Dean and Sam achieved adulthood; after this episode, we see Dean finally acknowledge and explore ideas or feelings he’d always previously denied, such as conceiving of himself as a father, or a husband—or, in this case, as a man who likes other men (or angels). Without his father’s shadow to tell him what to be or what to think, Dean increasingly begins to exhibit signs of bisexuality and, of course, “Casexuality” – until by Season 7 he’s not even really attempting to hide these feelings any longer. And of course, as I’ve spent a lot of time and energy arguing over the past few weeks, one of the basic assumptions of Season 7 is that Dean is in love with Cas; the plotline requires that the audience accept this as fact and move on from there.
Some fans argue that since Dean sleeps with women, even in Season 7, he must be straight, but that is a false equivalence. Sleeping with women does not rule out one also having sexual attraction to men. What’s more, one can prefer women at one age but prefer men at another (or both at the same time); sexuality is a fluid thing that shifts as we mature.
But sexual fluidity or bisexuality does not seem to exist in today’s social consciousness. Sixty years after Kinsey, we still view sexuality as a binary switch that’s flipped at birth, which nothing we ever do can change. You’re “born this way”, so you might as well deal with it.
But the truth is, sexuality is far more slippery thing than we acknowledge, and I have no problem with the idea that Dean’s sexuality might’ve shifted over time. Has Dean gotten “gayer” as the seasons have progressed? I think so, and whether that’s because he feels more comfortable expressing feelings that were always there, or because his sexuality is starting to shift now that he’s an adult, doesn’t really matter. Part of being a man is accepting that you feel what you feel, and although I can’t confirm anything for sure, I think it must be a hell of a lot easier for Dean to do that when he doesn’t have his father’s voice in his ear telling him not to.