Here's the scenario: assisted reproduction allows infertile people to reproduce. If the causes for the infertility are genetic, the genes for infertility traits will become more common. Eventually, after many, many years, they may become universal (since they stopped being barrier to reproduction). This might happen faster than expected if there's a strong selective pressure for genes related to infertility for other reasons, or the treatments prevent selections for adaptions to newer conditions (e.g. pollutants).