Yes, it is a sin, but so is a creep going around spreading his seed, or abusing his wife and children, or sitting at a bar drinking his rent.
A good solid family is preferable to this, but this is preferable to a screwed up "straight" family.
I would say at a certain point you just get to a "both bad" level. Anyway, to say that one thing is "preferable" over another is really meaningless. I would prefer a child be raised by a man who rapes women rather than a man who rapes children. But both are unacceptably bad. That's just one example. When you get to the bad-for-kids level there is no point in ranking the particulars. It's just bad.
What makes this extra bad is that it is a scenerio that never has to be. Lesbian sex never produces children.
Single women who resort to providing their child with a sperm donor for a "dad" truly cheat this innocent child.
The fact that some "on-the-scene" fathers (and sometimes mothers) deserve to be hung by their toes does not condone "sperm donor" fathers. Having a child by those means does nothing to help kids who have bad parents. It doesn't relate.