I think it would depend on the maturity of the person concerned and their ability to support, educate and look after the child should the prospective father renege on his promises, marry someone else, fall ill and not be able to work etc etc. (Yes, I am aware that this can happen in a marriage, but here one is choosing to be a single parent).
What are the expectations of this prospective father, what are his wants and needs, and how closely do they match those of the prospective mother?
Is it necessary to draw up a legal document to cover certain areas of responsibility?
What about extended family will they be there to give support should the lone parent need help, work late, be sick or have to travel for business? All questions to be thought about, asked and answered before another human being is brought in to the world.
Children are hard work if there are two parents, each child is a long term commitment with their own set of needs, ideas and plans, which don't necessarily agree with those of their parents!
What will the child be told - is it ok not to have married parents ? Divorced parents unfortunately are one of the norms of a child's life today, but parents who decided not to marry?
As I work at a school, and counsel children - I am aware of how much they want to fit in and be normal, 'like everyone else', The sensitivity of the child will obviously affect the degree of concern the child will show, and how it is expressed.
As much as I would love to help, this question can only be answered by the two people concerned.