Interviews with sperm and egg donors reveal an interesting dichotomy: while men who give sperm think of themselves as fathers, women who donate eggs don't see themselves as moms. This may say a lot about how we view motherhood and fatherhood.
The Washington Post reports on research by sociologist Rene Almeling, who says, "One of the most surprising things I found was that sperm donors have a straightforward view of themselves as fathers, while egg donors insist they are not mothers." She elaborates,
My research points to a long-standing cultural assumption in which the male contribution to reproduction is seen as primary. Indeed, the ancient Greeks, who thought of men as providing the generative seed and women the nurturing soil, would recognize a modern-day incarnation of this formulation in fertility agencies. Sperm donors think of their seed as essential to the child, down playing the role of the recipients. Egg donors insist that their contribution is "just an egg," pointing to the recipient as the mother, because she is the one who nurtures by carrying the pregnancy, giving birth and raising the child.
That part about "raising the child" may be key. Things are changing, but motherhood still carries far higher expectations than fatherhood. As non-custodial moms like Rahna Reiko Rizzuto have pointed out, women who don't live with their children are often considered bad mothers, or barely mothers at all. And so it makes sense that women who provide an egg but zero care don't think of themselves as moms. Dads, by contrast, get praise just for going grocery-shopping. If a man who buys the cereal is a great father, maybe a guy who just provides sperm is still a father, at least a little bit. The bar is set so much lower for fatherhood that it's no wonder sperm donors think they clear it.