In order to have a broad understanding of what homosexuality is in accordance with the teaching of the Catholic Church, we have to first know what the Church really teach about homosexuality. Homosexuality per se is not a sin which means a homosexual person is not living in sin unless he perform things that are contrary to morals. On the other hand homosexual acts are detrimental to the person’s salvation. When we speak of homosexual acts we do not refer to signs of feminine trait but actions or beliefs that are contrary to divine law such as same sex marriage or same sex unions. The Catholic Church has this to say about homosexuality.
Homosexuality refers to relations between men or between women who experience an exclusive or predominant sexual attraction toward persons of the same sex. It has taken a great variety of forms through the centuries and in different cultures. Its psychological genesis remains largely unexplained. Basing itself on Sacred Scripture, which presents homosexual acts as acts of grave depravity, tradition has always declared that “homosexual acts are intrinsically disordered.” They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity. Under no circumstances can they be approved. CCC 2357
The number of men and women who have deep-seated homosexual tendencies is not negligible. This inclination, which is objectively disordered, constitutes for most of them a trial. They must be accepted with respect, compassion, and sensitivity. Every sign of unjust discrimination in their regard should be avoided. These persons are called to fulfill God’s will in their lives and, if they are Christians, to unite to the sacrifice of the Lord’s Cross the difficulties they may encounter from their condition. CCC 2358
Homosexual persons are called to chastity. By the virtues of self-mastery that teach them inner freedom, at times by the support of disinterested friendship, by prayer and sacramental grace, they can and should gradually and resolutely approach Christian perfection. CCC 2359
In CCC 2357 the Catholic Church defined homosexuality as men or women who experience an exclusive or predominant sexual attraction toward persons of the same sex. In other words people who are attracted to the same sex. Furthermore, the Catechism also clarified that homosexuality per se is not a sin, but homosexual acts are. The Church also expounds the nature of homosexuality calling it as “objectively disordered”, this is no polemical language of the Church but a fact that is well established by science. This clarification is important in order to disprove the belief that a homosexual automatically goes directly to hell simply because he or she is a homosexual. In Diagnostic and Statistical Manual for Mental Disorders 5th ed. a manual used by psychiatrists to guide them in diagnosing mental disorders classified homosexuality as “Gender Identity Disorder” with corresponding signs and symptoms.
302.85 Gender Identity Disorder in Adolescents or Adults
Gender Incongruence (in Adolescents or Adults) 
A. A marked incongruence between one’s experienced/expressed gender and assigned gender, of at least 6 months duration, as manifested by 2* or more of the following indicators: [2, 3, 4]
1. a marked incongruence between one’s experienced/expressed gender and primary and/or secondary sex characteristics (or, in young adolescents, the anticipated secondary sex characteristics) [13, 16]
2. a strong desire to be rid of one’s primary and/or secondary sex characteristics because of a marked incongruence with one’s experienced/expressed gender (or, in young adolescents, a desire to prevent the development of the anticipated secondary sex characteristics) 
3. a strong desire for the primary and/or secondary sex characteristics of the other gender
4. a strong desire to be of the other gender (or some alternative gender different from one’s assigned gender)
5. a strong desire to be treated as the other gender (or some alternative gender different from one’s assigned gender)
6. a strong conviction that one has the typical feelings and reactions of the other gender (or some alternative gender different from one’s assigned gender)
To sum up the information above the symptoms of homosexuality involves the strong desire in doing or performing the roles of the opposite sex (i.e a man wants to fulfill the gender roles of women or wants same sex marriage), a strong desire to rid one’s primary or secondary sex characteristics (i.e a man wants to undergo sex change surgery), a person who strongly associates himself as a man if she is a female, or a female if he is a man. Given the facts above same-sex marriage or same-sex relationships are all signs and symptoms of gender identity disorder.
Therefore, to legalize these acts is nothing but legalizing the symptoms of a mental disorder. Aside from telling us of the nature of homosexuality the Catholic Church also tells us how to properly treat homosexuals, in CCC 2357 we are reminded that homosexuals are called to chastity which means it is commendable for them to remain celibate rather than engage into homosexual acts. The foundation of homosexual acts or homosexual union is not love but lust. We are also reminded that discrimination has no place in the Christian attitude towards homosexuals.