You don't have to buy a gift, although, as Toots points out, you need to evaluate what sort of message, if any, that will send in your family's context.
Do write a short note wishing them happiness. It doesn't have to be on a purchased greeting card. In fact, if they enclosed a response card with enough room for a few lines, you can write it right on that.
Do not give them any marital advice, and don't tell them what you do and don't believe in or approve of or how you feel about weddings, marriage, or gifts. Any of those things would be rude in this situation: no one is taking a referendum. They are just inviting you to share a very important event in their lives (that they DO believe in) and offering you hospitality. It would be just as inappropriate to tell them how you feel about weddings and so forth in your response as it would be to send a diatribe against religion in response to an invitation to a bar mitzvah or christening.
I, too, am taken aback at your calling them rude for expecting a gift and for that matter assuming they expect anything other than a response to the invitation. Unless there is something you haven't told us, and you are saying that just based on your having received an invitation, this looks like projection that indicates some pretty strong negative feelings on your part about these people.
Just decline the invitation politely and with a few nice words of congratulations and good wishes.