Sakusa's family is right. With his act he would never find an alpha. That's the point, he found a omega. It doesn't matter for them that others might not like it or that they can't mate. They are happy that they have just eachother. Only them No one else. Just Kiyoomi and Atsumu.
Being two omegas in a relationship isn't easy, but they managed. Coming out to the team was unexpected after one of their games in the locker. It was accidentally, but the rest of the team supports them.
They couldn't mate the same way Alpha/omega, so they got matching tatoos symbols of their love. They may not he traditional couple or very liked by society, but they loved being themselves - the most important.
In the end if the story, the two omegas were finally happy, the bought their own house, got mated in their own way. Also they have pups. The pups that Sakusa Kiyoomi- actually Miya Kiyoomi carried.
The finally got their happy end...