I think I tend to think about it myself just the same way. "Slash" to me implies that there is an original text in which this sexual relationship isn't happening. I think you could defintinitely describe an original piece as slashy--that's what original works are there for, after all, to provide subtext that can be made into slash. It seems like if there is an aesthetic part of that aesthetic is having an original text out there--though now I'm beginning to think maybe that original text could be imaginary!