[There's a somewhat awkward pause. Not because the idea bothers her, of course not; romance is romance. She doesn't see a problem at all with it, the thought that women could find love with each other just never occurred to her until now.]

I'm honored that you think of me well enough to tell me. And I really am happy for you both. [The awkwardness fades a bit, a fond smile replacing it.] You're both such kind people, it must be wonderful to have found happiness together.