I think it's a terrific idea.
It will make a special unbreakable bond between you & your friend.
And If your Girlfriend likes kids, then you'll have one that you can both take to the Zoo from time to time.
I can't foresee a single problem with the whole crazy idea.
Sperm Donation - Good or Bad? Page 6
N.B. be careful how you do the donation.
Carrying the spunk around in a water pistol and shooting it at women who look deserving will get you put on the sex offenders register (again I think I may have almost as many entries as SlaggA)