Yesterday, in my confirmation class, one of the students said her teacher (of a Catholic High School) said homosexuality is not a sin.
That is not true.
Homosexual activity is always a mortal sin.
Homosexual activism is a mortal sin, viz. scandal.
Any public recognition and promotion of homosexuality is a mortal sin: the sin of scandal.
Homosexual inclination is a mortal sin if willful, if unwillful yet culpable it is a venial sin.
Homosexual inclination (as with any inclination to sin) may be culpable or inculpable depending on whether it was/is caused by one's own willful sins.Homosexuality, a gravely disordered appetite, is usually a sin. Whether it is a mortal sin or venial sin depends on one's willful involvement in the depravity. If one should have the inclination without any culpability or willfulness then and only then it is not actual sin. But even in the case of an inculpable and unwillful appetite, the disordered sinful appetite itself is ultimately caused by sin--concupiscence--part of the inheritance of original sin.
Therefore, homosexuality is a sin, mortal or venial, actual or original, depending on the above distinctions.