Choosing debate topics is tricky.  I encourage learners to engage in debate on both sides of an argument, even when they don't agree with the view point.  I see this as a safe place to practice this skill, which can be very useful in understanding both your oppositions argument and view point as well as your own.  By understanding the other side, you can more fully understand why you may hold a specific opinion or position.  

Having said that, I also tell my learners that it is important to know your boundaries and to know when you simply cannot argue a certain position.  The topics are announced a week ahead of time, so if it is a topic that is unsuitable for the learner, they can opt out for that week.  As well, as a rule, I stray away from current political issues and insist that the class remains respectful to everyone's personal right to their own thoughts and positions.