Your marriage ended in divorce, therefore the relationship was a failure. Is this fallacious reasoning? If so what is the fallacy?

It appears that the person who wrote/said this, is not differentiating between "marriage" and a "relationship," which is fallacious. "Marriage" is a subset of "relationship" whereas all marriages are relationships but not all relationships are marriages. This would be like saying

Dogs bark too much, therefore animals are annoying.

While I can think of no named fallacy for this, it certainly is fallacious reasoning. If they had said

Your marriage ended in divorce, therefore the marriage was a failure.

They would have avoided the fallacy, but one could certainly disagree with them in principle.

Ryan Hardy pointed out that this might fit under Fallacy of Composition . I agree.