What do you mean, you don't agree? How you agree or disagree with someone being gay? That makes no sense. 
How do I do that? As simply as I posted above.
Doesn't make sense? All can do is provide a quickly googled defintion of "agree".....hope it's helpful.
a·gree (-gr)
v. a·greed, a·gree·ing, a·grees
v.intr.
1. To grant consent; accede: We agreed to her suggestion.
2. To come into or be in accord, as of opinion: I agree with you on that. Our views on the election agree.
3. To come to an understanding or to terms: We agreed on the price.
4. To be compatible or consistent; correspond: The copy agrees with the original. His story agrees with mine.
5. To be suitable, appropriate, pleasing, or healthful: Spicy food does not agree with me.
6. Grammar To correspond in gender, number, case, or person.
v.tr.
To grant or concede: My parents agreed that we should go.