Keep in mind that if you're donating eggs, you're allowing people who REALLY WANT children to have them, when other options have failed for them. You're doing them a tremendous, generous service. Although you get paid for egg donation, you are giving people the gift of a family. Ethically, I'd say it's extremely…

Why is it just infertile couples that have a moral obligation to adopt? Why not everyone?

Adoption in the US is nowhere near as easy as people make it out to be. And try doing a domestic adoption if you are gay, atheist, muslim or some other generally reviled group. Foreign adoptions have their own set of ethical problems.