Possibly 2 scenes referencing different anime but it's what I can recollect.

First scene: It's about a guy who has sex with 1 girl and his penis grows extremely big. They're in a darkly dimmed setting, smaller room and the actual sex scene showing his large cock is portrayed as a shadow on the wall.

Second scene: Group of school girls (I believe the swim club within the school, ~3-5 girls) find out about his large penis, find him at a swimming pool and start taking turns with him. At one point he tries to concentrate on math equations (nerdy, thought bubbles) to not get aroused and get hard/big. A red haired girl is the main initiator.

Thanks in advance for any suggestions and tips! :)