Platonic?! Are you fucking kidding me? Brothers fuck each other all the time. Read some hardcore Wincest fan fiction sometimes. Sure it's sexual. It's all about sex. It always was. That's what Wincest is all about: sex. Win-chesters + incest = Wincest. Sam and Dean are fucking gay couple. Platonic sex?! Platonic my ass!

It depends on whether you're asking about the canon, from-the-show relationship, or fanon. Clearly in the show itself, their relationship was intended (originally at least) as platonic/entirely brotherly, though once they learned about Wincest they also couldn't resist the occasional tease.