I hooted when I watched the exchange in the video below, because it reminds me of many I've had over the years. When I was teaching Natural Family Planning I often had people ask me about it. Then a few minutes into the conversation they would start attacking the Church's teaching. I'd scratch my head and ask, "Uh, who brought up this conversation? I'm not trying to force you to use NFP; I was just answering your questions." Then I'd drop it.
The reality is that a guilty conscience needs no accuser and oftentimes people are convicted by your existence if you are trying to lead an authentic, integrated Christian life. I think the best thing to do in these situations is keep your sense of humor.
When I was pregnant with my fifth, several people (at Church no less) said things like, "I thought you taught that birth control thing." At first I was offended but I knew I couldn't get through the entire pregnancy angry so I started laughing and replying, "Yes, and if you use the fertile time, you get a baby." Another woman at Church asked me, "What does your husband think?" That really made me laugh, and I ansered, "He was there!" An anti-life culture provides lots of opportunities to witness to the truth just by being who you are. A smile on your faith, love of neighbor in your heart, and a silent prayer in your mind cover a lot of awkward situations.