This is definitely a big question, and I don't think the answer is a simple one.
On an individual level, sex is only as important as you make it. If you consider sex to be an important, intimate act you would only do with someone you're very close to, that's totally cool. If you feel that sex is just a fun activity to do with a variety of people, then there's nothing wrong with that. What's important is that as long as all sexual activity is
safe, sane and consensual (and, you know, legal), we should not judge others for their sexual desires or activity.
On a cultural level, sex has always been pretty complicated and full of baggage (whether warranted or not). Sex has been used as a way of controlling people, and has been institutionalized in all kinds of ways. There is power in and around sex, and something that seems as simple as shaping cultural norms around sex (see below) can have a huge impact on a culture. There's a ton of theory out there, but there's still a lot we don't know about human sexuality. It's a huge topic that I have no hope of accurately summarizing, but if you're interested, check out Michel Foucault's
The History of Sexuality.
For individuals, the importance of sex depends on the person, but from a more cultural perspective, there is a great deal of power in sex, and it's hard to be entirely unaffected by that.
Obviously the latter half of the 20th century has changed sex from a means of reproduction to a pleasurable activity
This isn't true from my understanding. Humans have had non-reproductive sex for much of history, both heterosexual and homosexual (not even looking at places like ancient Greece). In Judaism alone, there are ancient laws about how providing pleasure is a part of the contract between husbands and wives.
It might be more accurate to say that non-reproductive sex has only become more acceptable in the mainstream in more recent history, though. Gayle Rubin outlines this with the idea of a
"sexual hierarchy" we have created, in which the most "morally good" or "normal" sex is between a heterosexual married couple who are monogamous, having sex purely for procreation, and are doing it in private - the rest is questionable or outright deviance. See also her
diagram of types of sex that are generally deemed "acceptable" VS "unacceptable" by society. The point is that we ask ourselves:
why do we think that way?
~Psychic