Watching other people's kids - WWYD

kiley&matt

Since I'm a single mama to an almost 1 year old boy, I sit home pretty much every night by myself, which I don't mind too much, because I love my son more than anything.  Lately, though, my other single parent friends have decided that because I'm always home at night, that it's okay to ask me to babysit for their kids EVERY WEEKEND and often during the week, too, so that they can go out.  I don't mind doing it once in awhile, but sometimes I'd like to just have an evening to myself on a weekend without other people's kids.  If I tell them no, not tonight, they want to know why I can't since I'm going to be sitting home anyway. 

I've tried to talk to a few of them about this, and they always reply that they will watch Liam anytime I want.  The problem with this is that they seem to always have plans, so any night that I decide that I'd like to go do something, they're already doing something and can't watch him.  What would you do in this situation?  I don't mind not being able to go out every weekend like they do, but I'm starting to feel bad for their kids, and I definitely don't want this to become a routine.