So there are several scenarios that I have in my head for how this could happen. 1) Sam and Dean were caught somewhere in a haunted gay club they were not supposed to be, so in order to cover for themselves they quickly pretended to be "IN LOVE." I'm a sucker for this "pretending to be gay" premise. Notice Dean's discomfort!face, where as Sam is like, I'm doing my best to be neutral. Bahahahaha. But then, boys, you realize touching feels good, and you can't stop.
The other is that Sam and Dean find the mirror of erised and this is what it shows. Bahahahah. Don't worry, Dean, Sam will hunch over so that you don't look so ridiculously small.
Well, wendy, I hope it lives up to expectation, and is better than the demonic!baby picture.
ETA: scenario three, that Sam from the future travels through time to tell Dean something, and is all hot and rugged, and Dean is still lithe and teenager-ish, and Sam can't help touching him. I think lyra_wing wrote a story to this effect.