The abstract mentions the following:

National-survey evidence on time use and sexual behavior is consistent with the iPhone reducing in-person interactions, increasing pornography use, and reducing sexual frequency.

But I would suspect it's less about pornography and time use and more likely due to easier and more private access to information about birth control and sexual health services, which is usually the primary factor influencing birth rates.

Men: fuck me please oh god I need to be fucked please fuck me

Women: I have a phone with internet porn now, so... No