Its okay to be in love with someone whose genotype is not compactible with yours if you both agree that you wont procreate.
This is a serious mistake most people make while going into relationships; they don't verify these things first and they are very important but there are ways around it if you have made the mistake and you cant bare the loss of your relationship.
I agree that it would be okey for them to stay together and decided not to have children or even adopt one instead.
But I doubt that is an option, since I have learnt from people living in Africa and also users on this forum that having children is an important part of the family culture there and not getting married and not procreating can lead to being socially rejected or stuff like that. In my opinion it is non-sense, since I am from the Americas; but I get how different cultures can be around the world.
I have learnt more about African culture since I joined this forum, tbh.
Adopting isn't a bad idea but you have no idea what joy it would bring for a woman to have her own child instead of adopting.
I can't see myself being with a guy I love so much and yet we can't get married, the shame it would bring to both of us when people finds out that we ain't married but we stay together, is like we're deceiving ourselves. Here in Africa things are done differently unlike yours, like you said people have different cultures and how they even live.