I don't find it analogous because even if a child with DS has loving parents, it will never be fully able to live in today's society. It's doable, sure, but it's not comparable 100% to 'normal' life. While being gay or straight does not impair your ability to live 'normal' life at all. By comparing the two it would be like saying being gay is a handicap.
That's fine that you don't, I am telling you 100% there are people who would. It's a matter of perspective.
Think about it another way - you can choose a male or female embryo. That is permissible under the law of many US states today.