I think I'd react the same way as I would if my children came out to me as a heterosexual: Slightly disappointed that they didn't identify as pansexual, but accepting of them as they are. My love for my children would not be tested or diminished by such a thing.
As a woman who has been sexually attracted to women as well as men in my life, I hope I wouldn't have much of a reaction at all...except to be supportive in whatever way I could.
Dd is 23 months, so we don't really have "talks" about things like that yet (and maybe we never will) but homosexuality is certainly part of our daily dialogue. Looking at photos, maybe that's the mama and the auntie, or maybe that baby has two mamas, etc. Dd has a friend who has two mamas, so same-gender parents are already part of her experience.
To me, it's no different than sharing anything else with dd. I talk to her constantly about all kinds of things, and I try to help her navigate images, experiences, things she hears and sees, things people say to her by talking about possible ways to interpret them.