I am surprised no one else has brought up the LW party line: consequentialism.

What is the alternative?
What is the consequence of your decision?

Probably the alternative is that someone else donates sperm. Either way, they raise a child that is not the husband's. If creating such a life is terrible (which I don't believe), is it worse that it is your child than someone else's? Consequentialism rejects the idea that you are complicit in one circumstance and not the other.

There are other options, like trying to convince them not to have children, or to get a donation from the husband's relatives, but they are unlikely to work.

If the choice is between your sperm or another's, then, as Qiaochu says, the main difference to the child is genes of the donor. Also, your decision might affect your relationship with the couple.