The difference is that Christ forgives.
To be forgiven, you have to repent. With a gang of 'helpful' therapists around telling him it's fine, how is he even going to know it's a sin, let alone repent of it?
Were that the case, only Jesus Himself would have been saved.
Jesus did not need to be saved. He is the Savior.
In fact, I know of no religion that suggests you can be damned for a single sin.
You must go to a more liberal church than I do. One mortal sin, yes just one, separates you from the grace of God, until such time as you repent, confess & are forgiven.
I really don't see how extra-marital sex qualifies as a mortal sin.
From the Catechism:
"1857 For a sin to be mortal, three conditions must together be met: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.
"1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother."132 The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
"1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart133 do not diminish, but rather increase, the voluntary character of a sin. "