Some of the earliest known art is erotic

https://en.wikipedia.org/wiki/History_of_erotic_depictions#Early_depictions

Evolution of sexual reproduction

https://en.wikipedia.org/wiki/Evolution_of_sexual_reproduction

Spewing sperm vs. nurturing offspring

Many sources make this case, but one of the best is Ogas Ogi’s and Sai Gaddam’s A Billion Wicked Thoughts (Plum, NY 2012). 

University of Montreal scientists couldn’t find any men who hadn’t seen porn.

http://www.telegraph.co.uk/women/sex/6709646/All-men-watch-porn-scientists-find.html

https://www.sciencedaily.com/releases/2009/12/091201111202.htm

Porn in brothels and private homes in Pompeii

Clark, John, R. Roman Sex: 100 B.C to A.D. 250. Henry N. Abrams, NY, 2003.

The Song of Songs

https://en.wikipedia.org/wiki/Song_of_Songs

Fanny Hill

https://en.wikipedia.org/wiki/Fanny_Hill

Lumiere brothers

https://en.wikipedia.org/wiki/Auguste_and_Louis_Lumière 

Porn during the Civil War

Lowry, Thomas P. The Story the Soldiers Wouldn’t Tell: Sex in the Civil War. Stackpole Books, Mechanicsburg, PA, 1994,

PornHub statistics, 2018

https://www.pornhub.com/insights/2018-year-in-review

Other estimates of porn use

Anon. “Where the Boys Are (Online),” New York Times, March 29, 2004.

Featherstone, L. “You and Me and Porn Makes Three,” Psychology Today, Sept./Oct. 2005.

Regnerus, M. et al. “Documenting Porn Use in America: A Comparative Analysis of Methodological Approaches,” Journal of Sex Research (2016) 53:873.

How much of the Internet is Porn?

www.covenanteyes.com/pornstats

www.internetsafety101.org

http://enough.org/stats_porn_industry

www.forbes.com/sites/julieruvolo/2011/09/07/how-much-of-the-internet-is-actually-for-porn

https://cyber.harvard.edu/vaw02/mod2-3.htm

Ogas, O. and S. Gaddam. A Billion Wicked Thoughts: What the Internet Tells Us About Sex and Relationships. Plume, NY, 2012.

Wright, P.J. “U.S. Males and Pornography, 1973-2010: Consumption, Predictors, Correlates,” Journal of Sex Research (2013) 50:60.

Femme videos arouse women

Beggan, J.K. and S.T Allison. “Reflexivity in the Pornographic Films of Candida Royalle,” Sexualities (2003) 6:305.

Laan, E.et al. “Women’s Sexual and Emotional Responses to Male- and Female-Produced Erotica,” Archives of Sexual Behavior (1994) 23:153.

Mosher, D.L. and P. MacIan. “College Men and Women Respond to X-Rated Videos Intended for Male or Female Audiences: Gender and Sexual Scripts,” Journal of Sex Research (1994) 31:99.

Quackenbush, D.M. et al. “Gender Effects of Romantic Themes in Erotica,” Archives of Sexual Behavior (1995) 24:21.

The best discussion of porn and romance fiction as two sides of the same evolutionary coin

Ogas, O. and S. Gaddam. A Billion Wicked Thoughts: What the Internet Tells Us About Sex and Relationships. Plume, NY, 2012.

Pamela, Or Virtue Rewarded

https://en.wikipedia.org/wiki/Pamela%3B_or%2C_Virtue_Rewarded

Romances as leading category of fiction

https://en.wikipedia.org/wiki/Romance_novel

Romances as sex education

Weiner, J. “We Need Bodice-Ripper Sex Ed,” New York Times, Jan. 21, 2018.

Compared with men, women experience more arousal from stories

Scott, C.L. and A. Cortez, Jr. “No Longer His and Hers, but Ours: Examining Sexual Arousal in Response to Erotic Stories Designed for Both Sexes,” Journal of Sex and Marital Therapy (2011) 37:165.