As a person who has attended GayDay (week) for the last several years, here are my thoughts:
I attend, usually with my now 81yo Mom, the first year I was a bit apprehensive and kept a watchful eye for anything that Mom may deem "inappropriate". Well, nothing "bad" materialized. Hand holding, pecks on the cheek, the "normal" interactions you see between people in a loving relationship, nothing "shocking". In fact, it is much easier to negotiate Mom's wheelchair through the more adult crowds rather than the young ones who are so overwhelmed with Disney magic they often are not careful of where their running little feet carry them.
As far as taking your 9yo DD, only you can make that decision, but bear in mind that more and more same sex couple are raising children, and many of them attend GayDay and have the same concerns for their children as you do for yours. AS far as "explaining" anything, if your DD has watched prime time entertainment shows on TV, she is probably already well aware of what "gay" is. Unfortunately the days of innocent shows like Bewitched and Beverly Hillbillies are long gone. She will take her cue from you, if you react in a calm, cool way like "so there are gay folks here, big deal", she will more than likely react the same way. If, on the other hand, your reaction is that holding hands or showing any affection is just awful and inappropriate, she will also react that way. Young people pretty much mirror what they learn from their parents until they reach the "terrible teens".
If you think you will feel uncomfortable, I would avoid the MK on Saturday. Other than that, at the other parks you may notice more people of the same sex that are obviously traveling together...but really, you usually see a lot of that at Disney any time of year. It seems that many people, hetero and gay, are putting off forming stable relationshiops until a little later in life in order to concentrate on their careers, and many people travel with friends of the same sex...it doesn't mean they are a couple.