Shambles define your terms here. Are we speaking of guitars as objects, the sound produced by a guitar and subsequent music composed for guitar or guitar players as opposed to banjos, banjo music and banjo players.
I would have to agree with sophocleese, above, if we are just talking guitars and banjos as objects, however there might be an element of sexiness, for some, reguarding the banjos taut stretched skin.
Traditional banjo music is not sexy, to me personally, on the whole, but there are a few traditional banjo songs that are exceptions and Dueling Banjos comes to mind. "Swamp Thing" by the Grid, is a very sexy, compelling piece of music, along the lines of Ravel"s Bolero, as an example of non-traditional sexy banjo music. But, as sexual turn on is a very uniquely personal thing sometimes and can be subject to gender differences, my choices are probably uniquely mine.
Regarding the banjo's percussiveness, I find this aspect of it's music can be quite sexy and the effect is often mimiced by flamenco guitatists, who recognize that quality of it's percussive sound. But like all things, if it is repeated over and over again, without let up or some dramatic tension, it can become boring.
As far as sexy players, I find most musicians, on any instrument, who become so into "it" that they start to drool, are very sexy, especially when playing what I perceive to be a "sexy tune", but that could just be a quirky thing on my part. *BG*