I can identify with Margot in this. As a young man I'd sometimes end up having sex that I felt awkward about, because I liked the other guy. Perhaps I was fortunate that none of these flings hurt the other guy enough to make them want to bite back even a little bit. Ships that pass in the night.
The situation she's in is a bit sad and empty, she doesn't see herself as ever finding the perfect soul mate she imagines. I recognise her conviction that "no such boy existed, and never would." When you're very young you can sometimes feel so trapped in the weight of your few prior decisions, and so blind to what you have to offer, that you can't imagine ever being truly loved.
As far as fat-shaming goes, her impression of Robert's physique is strictly private; we're being given a private view of the contents of her soul, she's not shaming anybody. It's not unusual for a younger person to find their older lover unattractively obese. I've spent enjoyable times with men whom I found quite physically gross. As I was young and gorgeous in those days (but couldn't quite bring myself to feel that way) I recognise Margot's enjoyment of the appreciation, the hunger for beauty, an older lover brings to the liaison. Sometimes that hunger we see in the eyes of a lover is all the validation we get.
Robert's ineptness as a lover, while comical, reveals to the reader his inexperience, which suggests that he's had problems approaching women. His awkward bout of robotic sex with Margot might well be the most wonderful and, in the end, the most soul-crushing sexual experience of his life. We don't get his side of the experience, which might make a different kind of story.
