The best environment for a child is the one where he is raised by his biological mother and father who are joined together in marriage.
When a couple adopts, they give the best alternative to an already broken household where a child can't be raised by his biological parents. When a couple with an infertile man goes to a sperm bank, they create a broken household because they are, by design, taking a child away from his biological father.
Adoption endeavors to fix brokenness. Sperm donation creates it.