Why are men and women so quick to become step parents but not quick to become an actual parent?

For example I have a friend who is 29 male, and he mentioned the other day that he is “ready to be someone’s step daddy.” And I hear phrases like this all of the time. However, for a woman like me who does not have children yet.. and I’ve been in a committed relationship for 2 years now.. my boyfriend won’t even consider having a child right now. He has dated women in the past who have children and that seemed fine to him. So, why is it so different when it’s your child? You’re still acting as a parent. I guess I just don’t understand why men are so okay with being with single mothers but aren’t okay with creating their own family?
Why are men and women so quick to become step parents but not quick to become an actual parent?
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