Assuming the sterilisation gene to be dominant (Deels being the forerunner to check its efficacy) would mean even fewer successful procreations through statistics alone. With fertile couples being a significant minority the chances of a fertile/fertile union is quite low and would take a long time to recover. The infertile gene will dominate the fertile gene resulting in an unsuccessful pregnancy.
But surely people would quickly find a way to beat the statistics! At the moment infertile couples don't (for the most part) just accept infertility - they try all kinds of things - as shown in this very series. I can imagine the few fertile men would find they could charge as much as they liked to act as sperm donors, and the few fertile women would be able to charge as much as they liked to be "surrogate mothers" to provide babies for the infertile couples.
And surely many governments finding their countries' population dropping alarmingly would make sure the fertile people got together in couples to reproduce whether willingly or unwillingly?