While it's always annoying to see the son 'all grown up' when he finally fucks his mom as opposed to her doing him when he was young, I love prostitute mom stories. For her to have sex with him without showing a care in the world because he's just like any other client to her, that's a special kind degeneracy.
the reason she didnt fuck him as a kid is both because she obviously never wanted to cross that line as even she had standards (that she in no way attempted to actually enforce she just assumed they would never be a problem) but more importantly its the same reason middle aged men cucking teenagers is so plausible. an undeveloped youth vs a fully developed man, in most cases there is no competition as the adult male is simply more physically imposing in every way. now that the son is a built adult he can outperform all those obese fat fucks that are past their prime.