This study is funny to me because there's such obvious answers to the trends:

Having less sex: more and more people are putting off marriage. I'm sure happily married couples are leaving even their most promiscuous peers in the dust with their nuptial regularity.

Rise in STDs: the risk of unwanted pregnancy is much lower these days then in say, the 60s and 70s allowing for "riskier" sex. Longer time to marriage + hookup apps + less risk of pregnancy = more partners, more chances for STDs to spread.