If a guy and a girl who are dating sleep in the same room but not have sex, is this considered immoral? If it is, what can I say to make them understand?
Your question leaves open too many possibilities. For example:
Generally speaking, for a dating couple to share the same sleeping quarters without sharing sexual relations can weaken their commitment to chastity and can cause scandal. It’s not a prudent idea. Here is the definition of scandal from the Catechism of the Catholic Church:
Scandal is an attitude or behavior which leads another to do evil. The person who gives scandal becomes his neighbor’s tempter. He damages virtue and integrity; he may even draw his brother into spiritual death. Scandal is a grave offense if by deed or omission another is deliberately led into a grave offense (CCC 2284).Scandal takes on a particular gravity by reason of the authority of those who cause it or the weakness of those who are scandalized. It prompted our Lord to utter this curse: “Whoever causes one of these little ones who believe in me to sin, it would be better for him to have a great millstone fastened round his neck and to be drowned in the depth of the sea.” Scandal is grave when given by those who by nature or office are obliged to teach and educate others. Jesus reproaches the scribes and Pharisees on this account: he likens them to wolves in sheep’s clothing (CCC 2285).
But, in individual circumstances, which can differentiate substantially from the norm, the possibility exists that the couple is not behaving inappropriately. Without specific information to evaluate, and without the reasonable assurance that we know the situation well enough to evaluate it, we should charitably presume that a couple is living chastely.
Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God’s forgiveness, it causes exclusion from Christ’s kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back. However, although we can judge that an act is in itself a grave offense, we must entrust judgment of persons to the justice and mercy of God (CCC 1861; emphasis added).