I don't think that there's any reference, in etiquette, that would agree with you.
I didn't say there was.
I said, I guess you missed it? It's not required.
But I think it's risky. Here's why:
This is the most important invitation this person will ever issue to you.
How you answer will tell them about how important they are to you (or how important their position in your family is to them).
You're not required to go, but how you answer will affect your relationship
When I save up money and time to go to my first-cousin-once-removed's wedding in Wisconsin, I tell her that she is important to me, and I tell her mom (my cousin) that SHE is important to me, and I tell my aunt that HER FAMILY is important to me. And I tell my MOTHER that her family is important to me.
If I decide not to go, even after having been told 2 years ago that Kelsey and Jack will marry when he gets back from Iraq and she graduates; even after having received a save-the-date six months ahead of time; I will be telling those people that they are not quite so important to me. That even with lots of notice and time to save up airfare, even though I don't need to rent a car, and can split a hotel room with my folks, etc., they're not high enough on my radar screen to bother.
If I give a reason--I just started a job and can't get tiem off; I lost my job and can't afford the airfare; my daughter is graduating from high school that same weekend--I can give some context to my decision not to go. I can change the message.
Now, maybe I don't WANT to change the message.
We get invited to every birthday party and christening from one couple in the extended family that we always decline, and never w/ a reason. We send a card of congratulations when the baby is born, but never a gift. And we don't send the baby a birthday card. Why? Because I'm hoping they'll take the hint and stop inviting us. I don't have anything against them, but they are not important to me; I don't want to reserve time in my schedule or my life, or money in my budget, for their family.
So, no, you don't need to give a reason. But all this communication (or lack of it) is how people send messages to one another about how valuable their relationship