It was once an offence to be homosexual in England.
Oscar Wilde actually did 2 years in prison for it. Then it became decriminalised. Then it became acceptable, with all the "we're here, we're queer, get used to it" campaigns. Then it became cool.
Now, in our post-PC society, you can actually be prosecuted if you discriminate against somebody for their sexual orientation. A far cry from Oscar Wilde's day.
However, some extreme viewed (right wing maybe?) Christians still hold this to be a sin, as the Bible tells us. Which potentially creates a nightmare situation where a Christian who is discriminating against a gay for being gay, could uphold his religious rights to find homosexuality offensive, and he is being discriminated against on religious grounds.
(A churlish conjecture here could be "how can anything be offensive to gays? Their idea of fun is a cock in the ass!")
Let's examine the more "right wing" Christian view.
The Bible tells us "men shall not give up their natural relationships with women and lust after each other." Look at Sodom. The whole town was at it. God sent two angels (Bartleby and Loki perhaps?) to stay at the house of Lot, "a righteous man". The men of the town heard there was fresh meat, and came knocking at the door, saying they wanted the 2 strangers to come out so they could have sex with them. (It's all in Genesis - check it out).
Lot had 2 young virgin daughters who he offered to the crowd instead. (Which is a pretty big deal when you think about it.) But the men were insistent.
Rather than having their angelic back passages violated, the angels blinded the men so they could all escape. Once Lot and his family were safely out of the way (except Mrs Lot who got turned into a pillar of salt for looking back when she was told not to - Big G was in a bad mood that day) the angels napalmed the town.
The town would have been dead in a generation anyway - a town full of homos will never bear any childen after all. But God wanted to make an example, as well as a point. ("Sodom? Sod 'em!")
So the town's name became synonomous with the act - Sodom - sodomy.
So let's define lust.
For a man it's erection and ejaculation. "Natural relations with women" is how a man inseminates his woman and how the species carries on. How ALL secies carry on. God gave us the power to create new life by this union of man and woman, this special sacred act. So one of His greatest commandments (though not on the list of ten for some reason) was "do not lust after each other". No man-on-man action. Do not put your seed into another man. You are wasting God's greatest gift.
(It could be argued His greatest gift is Free Will, but then you would have free will to commit sodomy.)
Of course lesbianism does not count as it does not involve insemination. As we learnt in American Pie, getting to third base means you are still a virgin coz you haven't actually had sex. Lezzas don't have "sex", just extended foreplay (though they will argue against this of course.) But lesbians do not commit sodomy.
Of course, this is just one point of view.