Simple question. Just imagine to be IN THE PLACE OF THAT INSTUMENT, regardless whether you're male or female. A banjo is a thing hanging from someone's neck. A guitar may be too but often it is hugged...EMBRACED...FROM BEHIND(!!!) Forget about the shape - that is only to make it more comfortable.
To guitar players: ever tried to handle someone the same way you handle your guitar? You can try the western style with its (her/his) waist on your right knee (easier to beginners) or the spanish style on your left knee (more acrobatic). Maybe it's not a good idea to use a bottleneck. DO NOT USE A CAPODASTER! Prefer fingerpicking to plectrum techiques but consider using picks (some like it harder).
A joke? I'm absolutely not sure. To me girls with cellos are very sexy (even when I see them carrying their instruments on their backs) and I remember of getting a little jealous when I saw my girl friend falling in love with a bassist (she ignored the guitar player completely). Just see that from the point of view I explained above...
Also, learning to play the guitar mostly urges you to learn singing, too, and that ist an additional factor. A guitar enables you to perform a song alone, without any additional accompaniment, a banjo much less. A banjo is played from inside a band, and only in bluegrass music its player is the 'primus inter pares'.