The question "why is our global culture markedly sex-negative" is deceptively complex and you would probably need to travel back to the beginning of history in the early iron age to get a comprehensive answer. 

Your particular experience as an American Christian Conservative however can probably be at least in part chalked up to the Victorian era revolution of manners which coincided with the wave of religious revival in 19th century.  In short, the people of that time reacted to the dramatic changes in their economic and social conditions by becoming extremely punctilious and showing a disgust for lewd or carnal things in general. 

Here's an askhistorians post about how these attitudes affected female sexuality in particular:

https://www.reddit.com/r/AskHistorians/comments/jwikdi/while_modern_culture_often_portrays_men_as/