There was this one with Shiori Tsukada and her "son" (together with other "mom"-"son" pairs) on an island/resort, and I remember it was quite romantic, but I wasn't able to find it now on my first search attempt. Will look some more later
P.S. Found it:
RCT-982 (what's confusing is that for this one, they don't use the term "incest", they use "relatives"... even though they are supposed to be mothers and their sons, and it's the "sons" who f... the "mothers", so...). Anyway, since it's Rocket, they might have made it into a circus show/challenge, so I'm basing this suggestion purely off my fond memories of Tsukada Shiori in this. Maybe I fantasized about being in such a situation with her and it stuck with me
There are two other "mother"-"son" couples here, of course, but I think we can all agree on who is the luckiest "son" of those three. If this one is actually any good, it's a series, so the other part(s) might be on par as well