The Question of Wedding Invites

The other day, a friend whom I hadn't met in a loooong time suddenly called me up and said that he had been trying to get me.
'Getting married?' I asked.
'Hey, are you trying to say I'll only call you if I'm getting married?'
'Yes' I replied - and got it right. He proceeded to sheepishly/elatedly invite me to his wedding.
Don't get me wrong, he's a great guy and all, and I'm happy for him - but what exactly is the protocol for inviting people to weddings?
Should it be confined to people you've met at least twice in the past year? People who've met both you and your partner? People who would potentially feel slighted if they knew you were married after the fact?
I get the feel that most Singaporeans operate under the last category. To which I would like to respond - NEWSFLASH - I will NOT get offended, I will simply congratulate you and be mildly relieved that I did not need to sit through a three hour dinner with people I need to make small talk with and pay $80 - $100 to go through such torture.
Of course, part of it is my fault. I could simply say - congratualtions man, but i don't think i can cos my pet salamander will be down with a cold..
But inevitably, I chicken out.
O well, another reason to buy evening dresses at least.