I know in my public speaking classes, we were required to at least mention the other side of the argument because it makes you more credible. What you might be able to do is use that tactic to turn it around. For instance, talk about the reasons why many women get abortions (financial issues, scared, feel alone, etc.) and then talk about programs or services the community offers to help these mothers overcome these issues (WIC, financial aid, counseling, donated blankets and bottles, etc.). You’d be arguing against abortion, but in a roundabout way that doesn’t negate the reasons women get them, which are often used to support abortion. It’s roundabout, but it might work.
Ugh. I hope that was easy enough to understand. My brainstorming tends to come out jumbled.
Anyway, I think it’s ridiculous that, on an impromptu speech, your teacher would be using such charged topics like abortion or gay marriage. I had two public speaking classes and both had an impromptu day. The topics were things like, “If you were an animal, what would you be?” or “Convince us that your homework is harmful to your health.” Charged topics should be used for full-length speeches, the students should be allowed to pick their position, and they should be allowed weeks of time to prepare for it. Any less is asking for trouble.