A friend of mine feels mono by default, and could be happy that way, but he got involved with a poly woman. He was open minded and willing to learn to accept her OSOs. He went through some discomfort over the years, but now he has embraced it just fine, and has his own OSO.
However, some people adapt to it, and some don't. IMO it is more common for someone to try and it decide to be mono (with someone else who is mono). Some monos stay with the poly partner, but they don't have their own OSOs.