A pre-op male-to-female trans-sexual who can switch between female and male roles in sexual encounters.
Sometimes also known as 'shemales', although shemales are purely a creature of the porn world.
Note - Not ALL pre-op male-to-female trans-sexual are, or even want to be, Switch Girls (and are not 'shemales'). The average M2F (male to female) TS (trans-sexual) just wants to behave and be regarded like a natal female, whether it be in bed or out and about living normally.
we spiced up our marriage by getting a switch girl in for a threesome, it was like having another boy or another girl in bed at the same time!