Is it OK to "go out"without your SO?

OK so my question is because my friends birthday is Sunday and she wants to do a girls night out tonight and I asked my SO of 12 years of it would bother him if I go and he said yes because he doesn't want to watch 3 kids 2 of them being his and my nephew and he is tired from working all week. I understand the being tired part but the kids are 9,7,5 and can do pretty much everything in there own. I would make them dinner and get them settled before I go so he basically just had to be here. I don't go out all the time and feel like sometimes I need a night out to myself am I being selfish or is he being childish? Honest answers please this is not the first time I've asked to go out without him and it always is a problem with him I don't get it!