I remember a while back seeing a hentai and I can only really remember 1 thing about it. The MC was with a woman. Then her sister, I think, comes into the picture and also wants to get with him. The original woman agreed, and when he tried to put it in, he was too big for her. And over the course of the hentai, over multiple episodes, she stretches herself out to get him in, and they eventually do it. And if I remember correctly, there was some bdsm/bondage type stuff going on throughout.