Earlier quoted context omitted.
Being straight has always been significantly more common, and will continue to be so. You teach them that when they grow up, they can marry someone they love. Which, now, is now true no matter whether you're talking to a kid who happens to be gay or not.
>>> You teach them that when they grow up, they can marry someone they love. That's avoiding the question. Do you, for example, show kids pictures of gay/lesbian couples and straight couples in equal proportion? Or do you show them mostly pictures of straight couples, and gay/lesbian couples only occasionally? Is it bigoted to teach kids that being straight is the "default"?
I would tell children what marriage is, as an idea, and when they ask who can get married you can say, "adults, or children with their parent's permission." It seems like you're concerned with fostering bias or bigotry in children. You will, we all will - our children will generally inherit our prejudices and politics. Part of raising children is working to create an environment where they understand how you feel, but also that disagreement is ok and questioning is ok.