That "all sexual intercourse between unmarried people is a (objectively) mortal sin" seems to be bedrock in Catholicism. When that is said, it is with the unsaid proviso that, as with all sin, one must be aware that the conduct or inaction is prohibited, the objective severity of the sin, and at the subjective level (God, sinner) the culpability may be mitigated or in fact non-existent.