This is a scene i remember from one with 2 or more chapters. I couldn't find it
There's the mc who brings his two gfs to an onsen where he does it with them, then some random guys appear in the onsen. Each time the mc switches and does it with the other girl(out of jealousy), the random guys do it with the previous one until mc passes out(i think) and the girls are left alone with the guys.
I think it should have been the 2nd or 3rd chapter.