always the mom

When there are 2 conflicting "appointments", or 3 or 4, depending on how many activities the kiddos have; why is always the mom who gets shafted? I have been a part of a book club for about 5 years now. It is with my twin mommies. We ALWAYS have our meetings on the second Wed of the month. That is all I ask for, one day a month. Tony plays basketball on Thursday nights, occasionally I have a conflict as well so we hire a sitter (expensive). But, Tony has been playing softball for work. My only request was as long as it is not on wed because I have book club. You guessed it, it is always on Wed. night. This wouldn't be an issue because Trystan can stay home with the twins. Well, for the last 4 months I have missed book club because of baseball practice, moving day and now of course football. His practice doesn't end until late so he can't watch the twins, mom who lives next door has plans so she can't watch the twins. Tony can't miss his game so he can't watch the twins. I am tired of having to pay for a sitter to go to book club when it would only be for an hour or so until Trystan gets home but at this point it is too late to find one as it is for tonight (my mom informed me just now she made other plans- when she was already planning on sitting). So of course, I, the mom miss out AGAIN. Why is it always the mom?