Can using a laptop on your lap cause fertility problems?

Probably -> Heat. Research shows that laptops increase scrotal temperature. Other research shows association of heat and and poor semen, particularly motility. If you want fertility then reduce heat situations like laptop directly on lap, hot tub, sauna and hot baths. See specialist for test and counseling. Fertility is a team sport so do not forget female factors. Best wishes.
Yes. If you don't get your laptop off your lap and pay attention to your partner you will have more problems than infertility.