Eruvin 100b - Menudah mikol adam

The agadata on 100b lists one of Chava's curses: מנודה מכל אדם, separated from all mankind. As the gemara and Rashi explain, she is forbidden to anyone but her husband, whereas a man is permitted many wives.

I am a little confused by this. The gemara in Sanhedrin 58a darshons ודבק באשתו: "He shall cleave to his wife - and not his friend's wife." This is the issur of eshes ish to a Ben Noach. And that was a commandment given initially to First Man. So even before the sin, wives were forbidden to other men. Why is it part of the curse?

R' Yitzchak Turk asked me an interesting question on my question. He asked, leave aside the gemara from Sanhedrin. Is there really a hava amina that, without the sin, women would have been permitted to have many husbands? What kind of world would it have been; no one would know who was the father of children? (And see Nedarim 51a on זימה.)
I answered him from Sanhedrin 38b: עלו למיטה שנים וירדו ארבעה - the children of Adam and Chava were (originally) born immediately, with no gestation period. There would never be doubt as to the parent.

A certain chacham suggested an answer to my question, based on the wording of Rashi. His suggestion was (as I understand it): Before the Sin of First Man, both men and women would never marry anyone but their spouses. They wouldn't want to: Adam and Chava were made for one another, born attached, so to speak. Monogamy was the natural state of mankind. But they weren't מנודה מכל אדם, because they weren't in any way deprived by that; the bond between husband and wife was so complete that they weren't missing anything.

After the Sin, the bond was weakened; the women was distanced from the man. He was now able to want to marry others, and was able to do so. She, on the other hand, though her bond with her husband was no longer complete, was unable to marry others: מנודה מכל אדם.

One thing that I think is interesting about the chacham's answer, is that it reminds me of people marrying their twin sisters. We see this various times in Midrashim, such as Kayin and Hevel, and the twelve sons of Yaakov - roughly, when there is no one else to marry. My question was, do they marry the twin born with them, or a twin of the other brother(s). In terms of the normal way עריות works, I guess I would have thought that they marry as far away as possible: some other brother's twin. However, that isn't true. We see from Kayin and Hevel, that Kayin was jealous because an extra twin was born with Hevel. That only makes sense if Hevel was the one to marry his twins.

It all just reminds me of Adam and Chava: Sometimes Hashem designs someone's spouse, as if they were created together.