I understand where you are coming from as, in my opinion, such topics should be left up to discussion by mature members only.
However, if you ban such topics from discussion, eventually it will lead to future problems. Members will start asking "Why can't I talk about religion?" or they will start complaining that you're "limiting their freedom of expression". How far will this go? Will any reference to religion be "forbidden?"
If not, how will you control what is or is not "allowed" to be discussed about religion?
If so, people will be getting in trouble for just mentioning something related to religion and you'll see quite an outcry for this "rule" to be struck down.
Aside from making a separate section made solely for the purpose of discussing "Mature" topics and regulating who has access, I don't find this suggestion realistic. Trust me, I'm tired of those threads too, but this does not seem like a feasible option.