@ulgaming my guess is they were doing a male only triathlon but with one difference: Busty and horny women would wait at key areas to seduce and have sex with them, most of the men falling victim to the seduction, save the main character, who had a girlfriend cheering for him, so he races to the end despite the over-temptation he was suffering from. He makes it in first place, and as a reward his girlfriend wants to have sex with him, but it seems that resisting all that temptation comically lead to Erectile Dysfunction (the last caption says "ED" which is the common abbreviation of Erectile Dysfunction)
Poor guy stayed faithful and now he can't get it up.
Well, at the end nothing is changed, when he gets home another pair of male boots at the entrance with sounds of sex in the background. He will never change, 27 chapters for the guy to realizes that's just the way he is, not a real NTR after all either lol