This is something I have sort of struggled with. Or, well, don't know if it's a big problem or anything, but I can't really make those boxes either, at least not ones I would completely trust. Often, if I share a really intimate connection with a friend, I feel there is sexual connection, too. There are some exceptions, I have a few completely platonic friends. But for me, I guess sexual intimacy is a really natural part of intimacy, and I feel like I want to share it (even as I don't always do that for various reasons).
I seriously have no advice to offer, though.