This is complex. First explain that we are called to love all people and to not make fun of or be mean to them.
Then explain that it is ok to point out errors if done in a loving way.
Let that sink in.
Once your child understands this concept you can introduce the idea that what some people do, even if legalized by the state, is wrong in the eyes of God. Explain that God made man and woman to fulfill different yet complimentary purposes. So if two men or two women get “married” it’s not right in God’s sight, so therefore a sin.
Last, remind your child that it’s never right to ridicule or be mean, especially to another kid who lives in a “family” headed by same sex adults.
Seven year olds can process quite a bit. Don’t underestimate them.