I think that guitars are generally associated with romantic mood music and banjos are not. The sensual music of the bossa nova, the French chanson, the Spanish classical guitar, the South American or Mexican romantic tune has a lot to do with the chordal harmonies and the lyrical melody played on the instrument.
Lyricism can be played on a banjo but is not often done. Generally, the sound is exciting and not conducive to the easy soft strumming of a guitar which in my view is generally sexier.
There are certain love songs that can be played on the banjo that might be construed as being sexy but even then, the instrument quality in it's timbre just doesn't quite match the lyricism of the guitar.
Violins and guitars tend to the kind of music that is considered romantic and sexy whereas the banjo and accordian and fiddles are fun, exciting and not particularly sexy with perhaps the exception of the French chanson or the Slavic and Russian tunes played on the accordian. One musical instrument style refers to candlelight and the other to bright stage lights or a rustic back porch.
Of course this is gross generalization. The biggest question has yet to be answered, sexy to whom?