I notice this not to be true with all... but very common with many people I know..

Why is it that some women in there past who may have had several sexual partners, unihibited sex, willingly experimented sexually, and even sought to have sex often... change this attitude once they get married / engaged... and they become conservative with sex, less intrested in sex, and don't seek sex as they once did when single / casually dating?

In such cases... what is the pshycology triggering this effect?

I can't answer with a fact but in my experience, it's the matter of we settle down, we don't have to get sexy, then we don't feel sexy, and then we stop thinking sexy. But in my marriage, my husband and I are very open with each other and keep each other guessing on what we'll do next! Hope this helps.

People grow, and change, and have other things on there mind besides sex. Wait till you have a couple of kids running around all day.

They grow up. They come to realize that sex is not the "happy pill" or the "magic wand" to get a man.
I don't think women become less intersted in sex when they get married, marriage and kids tend to slow it down a bit.
And maybe, she has not lost interest, the man is just not "all of that". He's not as interesting as he used to be.