Great question, Matthew...and not an easy one, either.
I suppose I would tend to imagine that the case of the married Protestant couple that contracepts is the more evil, since there is necessarily a conscious decision inherent in the relationship, agreed upon by both partners, not to reproduce, and to use the gift of human sɛҳuąƖity soley as a vehicle for physical pleasure, in spite of their marital obligation.
In the case of singles (paticularly younger, hormonally active singles), Catholic or otherwise, whose acts happen to cause a baby to be born out of wedlock (removing the possibility of abortion, for the moment), the scenario is probably one of two people who gave in to temptation in a moment of weakness (how they got themselves to that point is immaterial). There was surely no conscious intent to bring a child into the world irresponsibly, and obviously no conscious intent to prevent conception.
I think the gravity of the sin and the culpability of that couple, then, are not as great as that of the contracepting couple. The two singles sin together through immodesty, imprudence, and carelessness, whereas the first couple wickedly conspire together in favor of evil.