D.B.
Don't get offended or embarassed by people who are more screwed up than you are! Which is a lot of people! "Are you pregnant?" is highly intrusive from any stranger, religious or not. You can say "I'm not pregnant" but that's giving them private info. You can fib, but that makes you feel bad. You can say "That's pretty personal" or "I'm sorry, have we met?" and when they say NO, then ask why they would be soliciting such personal info from a complete stranger. If they go away, fine. If they persist, say "this is making me uncomfortable."
I don't think you have to go thru life avoiding eye contact with the world just in case there are a few intrusive ones. I think you just have to be polite but still be firm, and DO NOT let complete strangers change your opinion of yourself. By the way, size 12 is completely average.
I agree that, if you join this group, you already know what's expected of you. If you are interested in a church, find one that doesn't proselytize. If you're not interested, don't explore any.
I've never been approached in public, just at the door by the Jehovah's Witnesses. They don't identify themselves as such, they usually ask a question about what I think of the state of the world. Then once they hand me their magazine "Watch Tower" I know who they are. They are undeterred by the fact that I am Jewish and have a mezuzah on my doorframe. We also had a group of Mormons come to a synagogue service (they were more than welcome) but then they told us the reason they were wearing their name tags is that they always do so when proselytizing! We thought that was a little much - to come into our house of worship to lure us into something else.
One poster takes about the Boston Church of Christ and Middletown Church of Christ - I know nothing about them and don't know if these are the people who approached you. But be aware that there is something called the United Church of Christ which does NOT do what you experienced - so names can be confusing. Also, there are evangelical proselytizing groups that don't have that word "evangelical" in their name, and there are churches that do have the word "evangelical" in their name who do NOT go up to strangers. For example, the Evangelical Congregational church in my town does not go around evangelizing.
There are some groups from nearly every religion that do this. Annoying I know but they believe in it.
Just try to keep control of the situation and don't feel obliged to get involved in any conversation that doesn't interest you or seems way to personal. It's hard when you are caught off guard, but you also don't want to be negative toward another person in the park who might have a kid your child's age, or who is just making friendly conversation with no ulterior motive, you know?