Probably not the MOST embarrassing but I played against girls for one semester in high school. Our coach was female and divided people up by skill level. I think she did this so the very gifted and athletic people wouldn't outshine the others and make them feel bad but that meant a lot of the boys played in competition with each other which meant the only people left for me was this other boy who had to be 400 pounds and then all the girls. I played basketball with a 400 pound HS boy and a group of girls since they were in my skill level. I was never athletically inclined though I did have a good physique. I am just not coordinated is all. So what I would do is I would, and I hate myself for saying this, always go on the opposite team as the fat boy and steal his balls and guard him to make me look better against the girls. My God it is so embarrassing to actually type.
We also had to go bowling for one semester. My God I hate bowling so much. The shoes. The fake laughs. The fake fun. I used to bowl like a 40 and that was with me half trying. Near the end I would just wait outside for the 1-2 hours while everyone bowled. My teacher said I had to participate or else she would give me 0 credit but I always told her to just give me a 0 then because I'm not having fun and I hate bowling. Think she felt bad for me. She gave me a 100 in that class.
This was embarrassing but for a different reason which I'm sure some incels will relate with. The most cringey part of Gym was when we had to partner up with someone. I would obviously never have a friend to partner with so I'd get partnered with a random girl since my school had more girls than boys. But right before I would see all the girls frantically try and find a partner so they wouldn't have to be partnered up with a boy. Then when the unlucky one saw she would be partnered up with me I would just see her face and demeanor drop to an all time low. She would look so sad like I had just raped her Mom right in front of her. Those girls would never even participate. We would do partner squats or partner push-ups and they would always just try and get away from me. I remember one time we had to do a trust fall. The girl just put her arm on my shoulder and whispered, "Just tell her (the coach) we did the trust fall." and when the coach came around I said we had done it already and the coach gave us participation credit. I didn't even get to touch her at all. Really pissed me off.