It's best not to keep secrets from your significant other
Aaron doesn't want to have children but his wife Lena does even though she has not told him.
Lena wants to have a child.
Lena tells Aaron that she has decided she wants to have a baby and gives him a choice.
Lena stops taking her birth control but does not tell Aaron because she doesn't want him to know.
Lena and Aaron break up on good terms, so they can find more suitable partners.
Lena gets pregnant and Aaron does not want to keep the child so the two fight and end up breaking up over their incompatibility and Lena's lie.