Since I work in MarCom I understand the need about driving engagement. It really depends on where you want to go with this and what motivates and interests you as a moderator. Reddit overall, in terms of culture, tends to drive engagement through controversy.

In the case of the forum topic, you stated that your community driver was to "discuss all aspects of sex and sexuality from a Christian perspective". Centuries of religious dogma around sex have oppressed natural sexual tendencies and desires. In some cases this has skewed sexual behavior outside of the norms and in other cases it has repressed natural sexual urges into be a horribly, you are going to burn in hell dogma that leaves many Christians in severe conflict with themselves.

To drive engagement, and in many respects, help to serve this community, you have to find these conflicts and expose them to gain a healthy dialog. Through this you will gain members and increase engagement.

If you think about it, that is EXACTLY how our current dialog got started. Not by the poll, but through my feelings of faith in terms of a sexual framework being applied to my symbol of faith. In my case I chose to object, but I am the exception and not the rule, 90% of people would ignore the poll based on their level of conflict with a Christian Cross being a symbol of their Christian faith.