I don't believe it went beyond heavy kissing.
The dialogue in the preceding short scene makes it clear that the Prince cuts the encounter short and runs off. And then the Wife sings "Is that all? Does he miss me? Was he suddenly getting bored with me?"
The song "Moments in the Woods" is an interior monologue. There is no other character listening; the Wife is singing to herself. Like Little Red, Cinderella, and Jack in their earlier solos, she is having a private moment after a disorienting experience and trying to figure out what it meant and decide what to do next.
When you have just had sex with someone, do your thoughts focus mainly on the kissing? The question "And did I kiss him back?" would not be terribly significant if they had gone much further. She is just talking to herself so there's no reason to soft-pedal it or use euphemisms, and the song is characterized by her honest probing of her feelings.
The song is touching partly because it finds her finally re-committing to fidelity and discovering a new appreciation for her husband. But, in keeping with the ambivalence that runs through the show, it's interesting that the scene makes it clear that the Prince is the one who pulls back, suggesting that the Wife might have gone further if he had stayed.