I'm going to stick my neck out here -
It IS babysitting - if it is resented... through 15 years of marriage during which I babysitted our children - I was NEVER invited to go out, it was not my choice to stay behind... I was never offered a babysitter - and if I suggested it I would get the 'what kind of mother are you spiel' - big difference from the average situation I know BUT the point I make is if they can't afford a babysitter or if the OP is ALWAYS looking after the child without being asked or offered a babysitter (or at least offered equal recreational time without the child) so he can do something else - it IS babysitting or at least single parenting...
Smile! It makes people wonder what you've been up to
Engaged to my sweetheart, WaterWolf