?

Manga I know, which should match your criteria:

http://www.mangago.me/read-manga/pet_keiyaku/

http://www.mangago.me/read-manga/futari_no_musuko_ni_nerawarete_imasu/


And I would really recommend the amazing free webcomic Dragon Husbands ( https://tapas.io/series/Dragon-Husbands), because I love how they all are equal in their relationship, but they are four people and become a couple (is this the right word in this case???) pretty late.

I only know of one.... Ai to Makoto

http://www.mangago.me/read-manga/futari_no_musuko_ni_nerawarete_imasu/

We’re no match for satou kun
http://www.mangago.me/read-manga/satou_kun_ni_wa_kanawanai_sachimo/