First of all, you're making a sweeping generalization about all women and it's not true of all women. There are women out there that are more open sexual from the get-go.
Second, there are a lot of factors that can influence why a woman thinks/acts in the above ways. One of them is upbringing. Some are taught that sex is dirty or only meant for one man--the one they marry. Some are taught that monogamy is the right way to live and anything else is wrong. Some are taught that "sluts" are bad and it results in slut-shaming and trying to avoid being called a slut.
Third, watch movies and listen to music. How much of it is unconsciously showing us what is "normal"? Marriage between a man and a woman is what is shown--hardly a woman/woman or man/man marriage. What about women having sex with only one man at a time but men can juggle several women and it's "normal"? What about all of those songs where there is an emphasis on how bad nonmonogamy is but they don't really focus on what the issue is...deception and lying? All of these seep into our consciousness and if you aren't aware of any other way of life, you begin to think that all others are "not normal".
Forth, there's such a thing as mental maturity. You're comparing young women who might not have a lot of life experiences to older ones who have seen a lot more and experienced more in their life.
Fifth, if one is brought up in a religious house-hold that can also have a major impact in how a woman views sex. I guess that's part of the second point I was making.
For myself, it was all of the above of why there was that "180-degree turnaround". I was brought up in a religious household. I was taught that sex was dirty and shouldn't be talked about. I was taught that masturbating, porn, pre-marital sex, and sex outside of a marriage was wrong, wrong, wrong. I was taught that one should dress modestly. I still am "brainwashed" by the media in terms of what we should look like, how we should act, and what we should look for in a sexual partner. I was a very naive young woman (and still am in terms of life experiences) but I continue to mature and hope to do so until I die.
I am sure there are a lot of other factors that I haven't addressed but I listed the ones that created that turnaround for myself.