What's your line of thought that utilitarianism would lead to sterility?

If you have a choice between feeding your kids and feeding someone elses, usually the other person's needs will be greater than your own. Even if you want to make more utilitarians, much the same applies.

More generally, the "memetic hijcking" model diverts resources from genes to memes - often leading to compromised fertility.

How about "it's better to sound as though one has a highly intellectual system of ethics"?

Something like that probably explains a lot of interest in morality in general. There are also things like: look how much time I have to spend on things not involving finding my next meal.