This isn't about lacking common sense, this guy lacks the ability to see and conform to normal societal boundaries, wether this is a physiological/neurological issue, or an emotional/psychological one, it is unlikely he is going to improve without considerable medical intervention.
This is a long road, with no end, it requires constant vigilance on behalf of the partner, and considerable drive toward self Improvement and self discipline on behalf of the other.
I am 40 this year, and in a relationship with someone who has paraphillias, (not in regards to children) but it is something we both have to keep an eye on and requires me to have an inordinate amount of inner security and self assurance, confidence and strength.

These people damage those around them, not always intentionally, but often without remorse. And without that it is difficult for these people to feel inclined to change.

No one else can make this decision. But I would suggest this isn't a wise or safe choice for anyone.

Good luck.