A woman is 35 years old. She is unmarried. She wants to have a child. She realizes that her childbearing years are coming to an end relatively soon. The proverbial "clock" is ticking away. She goes to a sperm bank and looks at the profiles. She doesn't like the idea of picking the sperm of a stranger, so instead she decides to approach a male friend of hers and ask him to provide sperm.
Her male friend has no children of his own and likes the idea. He is interested in helping her. He wants to know what his involvement will be. Will they share the child and raise the child together? Will the child spend nights at each of their homes? Can they be like divorced parents sharing custody?
Her response to this is "not exactly". When asked what that means, she explains to him that he will help her achieve the pregnancy but he will not have legal responsibility to the child. He will be the child's "uncle" and he can take the child out once in a while on outings or to play, he can babysit, and he can celebrate some holidays with the child, but in the end she is the mother and the child's only parent. She tells him that this is a good arrangement for him because he never has to pay child support. He gets to see his child grow up and participate in that in a limited way, but he is also untethered and can do whatever he wants.
He thinks about it and says it sounds good to him. But what one thinks before the child is here, and what one feels afterward are often different. After a year of playing "uncle", this new father wants to be the legal father. If he has been involved enough with the child, he may be able to establish legal parentage and the new mother loses the control she had thought she would always have.
There's a simple principle at play here. It's the old adage: "You can't have your cake and eat it too". If you are a mother contemplating solo parenthood, and you are certain you don't wish to end up in a joint-custodial arrangement, it is not realistic to expect that your friend is going to abide by your wishes long term. Using an anonymous sperm donor solves this problem. And if you are the friend who thinks it might be cute to have a child that you don't really have to take full responsibility for, that's also not very realistic. Once your flesh and blood is in your hands, it's not so easy to say "he's not mine". And if you do start taking that active role, don't be shocked when you are expected to pay child support, or some portion of the child's expenses. It is expensive to raise kids.
These are issues that really need serious consideration and should not be decided based upon fantasy scenarios that sound like great ideas but which are both unrealistic given that they involve human emotions and have legal consequences. These arrangements have to be thought out in advance, probably discussed with a mental health professional and which certainly need the involvement of an attorney to draft an agreement that ensures that whatever the expectations are going into the arrangement, will also be the rules for moving forward once the baby has arrived.