Could it be https://www.mangago.me/read-manga/crossing_the_line_1/ but i could be wrong tho cause maybe its useops romance or whatever
I was so convinced I knew what this was. So I tracked down the story I thought it was – but skimming over the first few chapters again I can't see a scene like you've mentioned.
But for a uni setting story where one guy (at least at first) doesn't want a committed relationship (and wants to have sex with a guy mostly out of curiosity), there's this:
https://www.mangago.me/read-manga/rivalry_1/
Though I think I dropped reading it coz or was annoying me lol
So,the thing is i don't remember the title nor do i know the name of the author, it is a complete bl manhwa.Let me tell you some details about the manhwa. The uke is a playboy and the seme is a shy, ordinary guy,they are in the same university,also the uke is a playboy and he is rich he sleeps around and i think he said something like he doesn't wants to tied in any kind of commited relationship, there is a specific scene i have a vague memory of ,it is from the restroom of the university the uke accidentally peek at seme's genitals and he was impressed by the size of it i guess and i don't really remember if the seme already professd his love for uke before this scene but I do remember that the uke decline the seme's confession but he wanted to sleep with him because of seme's d*** size,and also
The seme fell first but the uke fell harder.
So yeah, this is all I can recall from my vague memory of the manhwa.
Sorry for my terrible english and pardon my grammar mistakes.