any yaoi manga where the uke is taking a shower and then the seme enters the bathroom and ...

That happened in one of this manga's chapters
http://www.mangago.zone/read-manga/punch_up/ and in this one. http://www.mangago.zone/read-manga/kuroneko_kareshi_no_asobikata/ and at page 128 of this manga
also started a d bath sex scene. http://www.mangago.zone/read-manga/k_sensei_no_yajuu_na_aijou/

http://www.mangago.zone/read-manga/kimi_de_oboreta_ato_wa/

http://www.mangago.zone/read-manga/my_life_with_you/mp/manga/My-Life-With-You/v1c4/20/