Congrats on your engagement.
I got the asked the same question by people when I announced my engagement years ago, and get similar type of answers. When I told people that I was not pregnant, and not planning to get pregnant anytime soon, some would often get a sour grape look on their face and ask, "Why bother getting married then?"

I noticed the question was usually asked by single parents (never married and divorced) with a "parter" (not a fiance), or by other people who later revealed (out of earshot of their spouse, of course) the only reason they got married was because there was an accidental pregnancy involved, and they "wanted to do the right thing".
And if you have a small, informal wedding because you would rather spend your money paying bills, downpayment for a house, etc., some people absolutely insist the only reason you are being so "cheap" is because you are pregnant and are getting hastily married.
It was interesting to watch the reactions of people when they found out about my non-pregnant wedding. I even had one cousin at my wedding reception break down into tears, then yell at her boyfriend how unfair it was that I got married before she did, when I didn't even provide her man with three children already.
*sigh* Whatever just happened to, "Congratulations! I'm so happy for you!" and just leaving it at that? And do these people know how rude it is to imply that "The only reason he would ever marry her is because she got knocked up"?
BTW...after 12 years of marriage and no children, the pregnancy questions never end. At this point in time my relatives are acting like I'm pulling a fast one on them because I'm approaching middle age with no babies yet.