In micro economics, when a substitute reduces in cost and increases in quality, we see a substitution effect [0] (amusingly, coined by economist Eugen Slutsky), i.e. people switch away from the relatively expensive good and into its lower cost substitute.
The primary substitute for physical sex is DIY with porn. Over the past 30 years porn has become orders of magnitudes cheaper (pre internet, it was ~$9 to see ~5 humans nude a la playboy magazine, and involved a trip to the shops; now it's approximately free and instant). Porn simultaneously increased orders of magnitude in quality and variety.
If you apply the same thinking to any other two substitute products (i.e. one gets 1000x cheaper whilst improving in quality and variety, whilst the original stays approximately the same), we could expect the exact same results, i.e. switching from one to the other.