Religion was and is not an earlier, superseded prototype of science. That's the Dawkins/Dennett view and it's just drivel. Popular drivel, but still 100% shit.

I wouldn't overdo the rationality side of things. Rationalism creates moral monsters.

In a world where there are two many people, it may be that the natural purpose of the sexual organs becomes recreational and non-reproductive in a large number of people. Certainly some animals change and stop reproducing, when confronted by over-population. Studies in mice show this.

But for this, if you are a Christian of a certain type, you may need a brand new Testament!

Or you could use the Catholic 'primacy of conscience' doctrine as a let-out. But it seems obvious to me that having ten kids with the planet as it is currently, doesn't necessarily seem very moral.