In general society, the concept of "sin" is almost never mentioned. In our post-modern world there is no sin because [just about] everything is permitted.
The only time you hear sin is in relation to dessert....
I saw this on a "open orthodoxy" website [written by a "Maharat" - sounds more like the name of a Rishon than a pseudo-Reform female clergy person]:
It was a parenting moment that came much sooner than I thought it would. My six year-old looked over at me at the dinner table and told me that her teacher had said that a boy “can’t marry a boy, and a girl can’t marry a girl.”
I paused, chewing.
“Well, what do you think?,” I asked her.
“Well, I know that isn’t true.”
She knows that isn’t true because we have had gay couples at our Shabbat table. She knows it isn’t true because she has a friend with two moms, and because her little sister has a boy in her class with two dads. She knows that sometimes boys marry boys. She knows that gay people exist. This is 2016.
I responded by reminding her (and my two- and four-year-old, who were also at the table) what I’ve told my kids many times: It’s true that MOST of the time, a boy marries and girl, and a girl marries a boy. But SOMEtimes, it does happen that a boy marries a boy and a girl marries a girl.
I think that this is a subtle form of child abuse. Not physical poisoning but spiritual and emotional. I must be fair - she doesn't mean it. Her intentions are noble but that doesn't justify poisoning a child's mind. To marry a member of the same gender is a sin [I am not referring to the marriage ceremony per se but to the physical relationship that marriage entails]. Why can't people say that? If you believe in G-d and you believe that there is moral code that He established, then the notion of sin must be central to one's world view. This "anything goes" attitude is a destruction to the world.
Her answer should have been "Yes, usually man marries a women but sometimes a man marries a man and a woman marries a woman. But that is against the Torah. Hashem gave us a mitzva to marry someone of the opposite gender - man to woman. That is also the only way to have biological children. Hashem wants us to have children in the framework of marriage and that can't be accomplished when a man marries a man. The purpose of marriage is to bring Hashem's presence into the home and that doesn't happen if people are living against Hashem's will. I am very sad for your friends with two moms and two dads. People who marry people of the same gender have much higher rates of depression and suicide. I hope that when grow up you will marry a man and have a beautiful family. I married your daddy and that is how you were born!:-)".