You're assuming memetic information propagates in the same way as genetic information. Not eating or having sex has been used to great memetic success historically.
Even if secular people went extinct today, there would be more 30 years from now. Religious communities can produce secular individuals, from which secular communities form that generate secular memes.
They operate on independent planes. Unless a sort of direct genetic disposition to specific ideas can be established, I believe the complexities of genetics and memetics allow for such a broad range of complex interaction that they cannot be correlated so easily.
Now that contraceptives exist, people who choose them and deliberately avoid ever having children will select themselves out of the gene pool. The same goes for people born in religious communities who are skeptical of their religion. The people who remain in those communities will be the ones who have some sort of genetic predisposition to religious experiences. This is already a hypothesis (the God gene [1]).
It may be that as behavior driven by memes versus genes diverge enough, the population naturally falls. That could be the solution to the Fermi paradox, that sentient population growth is limited by the growth of their meme pools, as they diverge in purpose from their gene pools.