Tennis Yaoi where uke hates seme because seme is better but seme is in love with uke?
Not exactly tennis but...
http://www.mangago.me/read-manga/aisaretaino/
Any yuri manga that’s more smut focused?
http://www.mangago.me/read-manga/i_fell_in_love_for_the_first_time/ (not strongly recommended)
Do you guys remember a Yaoi where there were these rivals and to get them to stop fighting or something, some girls suggest that the uke should try and act attractive to the seme so that he would get grossed out. However, it backfires, if you know what I mean
http://www.mangago.me/read-manga/sailor_danshi/tr/97949/page/55/
Hopefully it's this one

Im looking for a BL isekai where the main character gets transported to a fantasy world where they think he’s the demon king (he isn’t) and the king or prince falls in love with him. The main character is buff and gets long hair