As someone who benefited from many years of fundraisers for chorus, dance, field trips and the like, I'm sympathetic with the difficulties schools face in raising funds, and also the difficulties families face in figuring out how to sell the requisite number of cookies or candles. However, there's no reason that you should single-handedly feel responsible for all of your colleagues' kids' extracurriculars.
When I was a kid, I was allowed to try to sell fundraiser items to anyone my family or I knew, but I had to do the asking myself. As an adult, I support this practice pretty strictly. For example, I only buy Girl Scout cookies when the scout herself is involved in the sale and not when a random adult sets up a stand in Rittenhouse Square. One option to get yourself out of these situations is to say, "I'm happy to support educational causes, but I make it a rule only to order directly from the students themselves." You risk your colleagues bringing their children to work, but, for me at least, that would show enough dedication to the cause that I'd probably be swayed to buy some caramel popcorn.
You could also decide that you have a budget for the year. You can spend, say, $50 on such fundraisers, and after that, your answer can be, "I've already reached my limit for the year on this kind of spending, but next year I hope to be able to support your son or daughter." You do, in fact, have the option to decline all such sales and to tell your colleagues, "I'm sure your kids' school will do great things with the money, but I'm sorry this isn't in my budget," or, "I'm sorry I can't help right now," or even just, "No thank you." As much as you worry about the ramifications for supporting some but not all of the causes, if you're more inclined to eat chocolate than cheese, then buy the thing you would actually enjoy and tell the next colleague that there's nothing in the catalogue that you need right now. I also suspect that even if your colleagues are keeping tabs on what you buy, they're probably not comparing notes with each other.
As for the Catholic school piece of the equation, if that's really the sticking point, then just make the decision not to get involved and use any of the above lines to decline politely. As much as this clearly feels like a peer pressure situation, you're really not required to support your co-workers' kids activities. I would say, though, that you can't avoid supporting fundraisers for Catholic schools while supporting fundraisers for public schools or Jewish schools. If you would consider distinguishing along those lines, you're better off not buying any of the stuff and finding another way to support causes you believe in.