very very long shot but this is the only manga ik where they have sex outdoors and there's a creep involved (but i dont remember him doing the stuff you mentioned) :/
https://www.mangago.me/read-manga/public_sex/
Hi there. Someone found the manga: https://www.mangago.me/read-manga/kizudarake_no_hakoniwa/
Is this the one you’re looking for? Coz this is the one I was thinking about in your post
Honestly I only remember one scene so it's unlikely someone will know this but it's a Japanese black and white manga short like 5-7 chapters they are both students high school or collage I don't remember but in the one scene the uke comes outside and encounters a creepy pervert he's taken into the bushes ( I'm pretty sure it's in or behind bushes) and the creep makes him masturbate infront of him the uke does it and then starts thinking about the seme while doing then the seme shows up and sees him doing it can't really remember what happened next though but yh
Possibly but I'm not totally sure they previously had sex outdoors and this creep had a picture of it showed him and dragged him off but that I could be mixing this bit up with another manga