It's most likely due to the frequency of sex in the media, which itself is largely due to the fact that birth control and protection now exist. Before, if you slept with people before getting married you had to worry about getting pregnant or impregnating someone or getting syphilis or something. The advent of condoms and the Pill meant that people no longer needed to worry about that sort of stuff, and thus it was more common for people to sleep with each other before marriage. That eventually trickled down to a loosening of morals. I don't think it can really be avoided, which is why we need to drill it into people's heads that casual/extramarital sex without condom use is *wrong.*