If that bothers you, then I wouldn't suggest looking up fanart, especially on Tumblr.
I can't explain it, but like Ahsoka said, Star Wars, especially is an iconic and extremely popular series. It's like how I loved dressing up as a Hobbit because I love the world Tolkien created. Living here, we all now know that anyone from something we think is fictional can be real. The locals, well they don't know that. It's like how I've seen costumes based off of my family, and I've seen some fan art too.
People love the story and the world you come from, and dressing up is one way how they express that love.
As for the sexy...that pretty much just happens with anything. There's always a sexy option.