The author of the following article from Huffington Post has some good words of caution on trying to use a sperm donor without the help of legal or doctor's advice:
Choosing to use a friend as your sperm donor is a big decision. Do not shy away from formality and transparency! You likely chose your donor for more than his pretty face -- you probably share a deep emotional connection with him. This makes transparency and formality that much more important. A contract ensures that you, your partner, and your sperm donor have a complete meeting of the minds regarding expectations, roles, legal rights, etc. The same holds true for involving a physician; it not only ensures medical safety but protects your donor legally.
What did you think of her advice?