I think the point of writing a scene in which one character desires another is entirely about seducing the reader. The reader needs to be drawn in, to identify with the desire to one degree or another. That identification may have little or nothing to do with anything the reader wants to do in real life, but it should be something that creates a fantasy for them.
That said, there are myriad ways to make a character sexy to a reader and not everything that works for you will work for every reader. Personally, I like to see the person who is the object of desire through the protagonist's eyes. I want to know what his/hir/her voice sounds like, how they smell, how they might taste. I want to see the protagonist long for them until I can feel their response to their fantasies.
In order to make that happen, the protagonist also needs to be one that a reader can identify with to some extent. Otherwise, seeing through their eyes is a less effective technique. This is not the same thing as writing the story in first person, something many erotic authors opt to do in order to create that immediacy for the reader. It’s important to remember that immediate to you is not necessarily the same as immediate to anyone else.
Making your point of view character sympathetic can be trickier than doing the same with her/hir/his object of desire. First versus third or second person is not a guarantee. Neither is making that person model/actor gorgeous and indefatigable. Writing the character as a complete human being, warts and all, on the other hand, gives the reader something to relate to. Who hasn’t wondered whether or not their object of desire is interested in them? Giving the character some insecurities and some sort of life apart from their desire goes a long way toward building a good protag.
Erotic writing should, at its best, call forth a strong reaction from the reader, even a negative one. That's its purpose. To merely describe one character wanting another without letting the reader feel that through your words puts the story outside the realm of erotic fiction, for good or bad.
KB: Catherine is the editor of Haunted Hearths and Sapphic Shades: Lesbian Ghost Stories, due out in May, 2008 from Lethe Press. I had the pleasure of meeting her at Saints and Sinners last year.