I have a personal policy:  I don't go to birthday parties for kids.  I'm a damn adult.  Why would I want to sit around with a bunch of screaming, snot-faced kids, and eat cake and watch cartoons?  Actually the only thing I don't like from that previous sentence is the presence of children.  I'd eat cake and watch cartoons all day, but I don't need to watch some stupid kid unwrap presents in order to do so. 

When I share my policy with other adults, they pretty much universally agree with me.  But there's this underlying feeling of obligation that I simply don't share.  I'll put it this way:  I don't remember the presence of a single adult at any birthday party from my childhood.  Your feeling of obligation is manufactured.  Just don't go.