I think I know where I'm going with this. Dark, butch-cut hair signifies masculinity. Long blond hair connotes a level of effeminacy. A clash is bound to pack some erotic heat.
That's mostly it. But then there's something else, perhaps related, perhaps not.
The short haircut looks conventional, suburban, orderly. Long hair looks untamed, risky, up for anything. Given this characterization, the short dark hair seems uptight, mainstream, perhaps even a little repressed: the picture of prudence and playing it safe, by the rules, missionary position, and the long blond hair seems wild and randy, in the dunes, under the open sky. Now the heat is primal and psychological--civilized social norms versus natural instincts--the superego versus the id, Apollo versus Dionysus. The hair color adds to the significance: brunet = serious, blond = fun. The struggle between the two is the struggle of law versus instincts--perhaps an offshoot of the masculine/feminine dyad.
Something else, too: the conservative shorter haircut suggests age and experience, the shaggy long hair suggests youth and vigor. Perhaps the self-disciplined pedagogue subduing the wayward ephebe?
What doesn't quite fit is the skin coverage. The longhaired blond should be more scantily clad, the shorthaired brunet's gear should be the more conservative, if only a little bit. The colors are right: Brandon's basic black, square-cut, Kevin's streak of vibrant color (pink or purple might have been better for my argument) ... with an arrow pointing to his butt-crack no less!