What seems to me to be the case is that male singers are much more likely to play guitar, and often regard accomplished guitar playing as more important than fine singing. Far more women sing unaccompanied, or accompanied by someone else on guitar.

So yes, bands are often predominantly male, but that's because of the instruments. Choirs (except for exclusively male-voice choirs) are always female-dominated. Put simply, far more women than men choose to sing, alone or with others, and consider the voice their main instrument. Far more men than women choose to play a guitar or other instrument and sing with it.

Marje