This is a question from my Bible class. Is being gay or a lesbian a sin?

No, but Leviticus 18:22 says "you shall not lie with mankind as you shall not lie with womankind." this means that god forbids male on male incest.

Yes, its a sin to be gay or lesbian.

Explanation:

God made two genders, Male and Female.

In the bible it says If a man lies with a male as with a woman, both of them have committed an abomination; they shall surely be put to death; their blood is upon them." Leviticus Chapter 20 verse 13

and another verse You shall not lie with a male as with a woman; it is an abomination." Leviticus Chapter 18 verse 22

So yes, it's a sin to be gay or lesbian.