A mortal sin requires three components:
- The sin must be of “grave matter”
- Complete knowledge of the sinful nature of the act at the time it was committed.
- Full and complete freewill consent to this act.
If NONE of any ONE of these three criteria are not met in ANY degree, then the act is not sinful in any way (venally or mortally). So a person who is compelled to commit an act which is normally considered sinful (gun to the head of his daughter, etc) which he would never otherwise commit is not culpable in any way for the sin, because he had NO consent whatsoever (the Church does not require heroic consent, but ordinary consent).
Venial sin is what happens when a person commits a sin which possesses all three attributes of mortal sin, but any ONE of these attributes are not complete and perfect. If our knowledge is 99% (not 100%), then the sin is not more than venial. If our consent is 99% (but not 100%) then the sin is not more than venial.
There is NO SUCH THING in Catholic theology as an act that is always mortally sinful, or always venial, or always inculpable. Because the act itself is only 1/3 of the equation (and thus can never be a majority of the culpability). People come to this Forum all the time asking, “is such-ahd-such” a mortal sin. The answer is ALWAYS the same: Maybe.
The same answer applies to venial sins. And only God knows for sure.