Yoneda Kou's Doushitemo Furetakunai and its sequel, Soredemo, Yasashii Koi o Suru, I guess. I'd also like to recommend Blue Sky Complex but it's ongoing...
This one is based on a real couple:
http://www.mangago.zone/read-manga/tatoeba_konna_koi_no_hanashi/
This one allowed a switch at the end:
http://www.mangago.zone/read-manga/metro_dog/
I'm not sure how realistic this is and the drama is on the lesser side, but the ending is super happy (like so happy I cried reading the last chapter): http://www.mangago.zone/read-manga/rutta_to_kodama/
A realistic yaoi manga with drama that has a happy ending?