While some controversy is fine, subjects should generally avoid the most extremely controversial topics such as abortion or politics. Such issues often have no middle ground on which agreement, friendship or understanding can be based. They therefore make for poor pre-evangelistic topics. The discussion is not intended to preach the gospel. The main point is to show interest in understanding other's views, sharing our own, and demonstrating that we are funny, thinking, friendly people.
Facilitators often visit other groups and offer their discussion there.