I think there might be a little truth in both. It's not that I think Sara necessarily got turned on by Libby and Goo Goo (Ga Ga? their sexcapades put a whole new shade on his nickname now) but if she's a virgin, it might have made her curious what would drive people to copulate in an alley. I don't think she's uninformed about sex but it might have been the first time she witnessed it.
And then there was John who both cares about her and was caring for her. Why let crisp, clean sheets go to waste?
If she's a virgin, which I can believe she is, I could see her being curious and figuring John was as good as anyone else to have that experience with. Overall, I don't think she was overcome with uncontrollable passion as the actual act, as filmed, was quite deliberate and tender.
I don't think Sara gives a hoot about standards. But I do think Violet was trying to "trap" John. An honorable man would marry her after taking her virginity even if she isn't pregnant.