"River Song I could bloody kiss you" – "Maybe when you're older."
Amy still has her eyes shut. She refuses to open them even after the Doctor has said to her a billion times that the Angel is gone from her mind, it never existed. But she's not deaf; after they get the gravity back, she can hear the kissing. She can hear the smack of their lips and the telltale roughness of their breathing.
She's jealous. Not just because River Song is kissing the Doctor, but because she can feel the heat of the Doctor's body against her own body and because River's hands are – without a doubt – reaching for the Doctor's shirt and pulling it out of his trouser's.
From the moment River had fallen onto the Doctor in the TARDIS, Amy had known there was something there. Something. And she'd called him "sweetie" – you don't do that unless he means something important. But she wasn't expecting them to end up kissing after that. Sure, in the heat of the moment, she would have probably kissed the Doctor too, if she hadn't been too scared to open her eyes and cross to where the Doctor stood. But after what River had said about him being "too young", Amy hadn't been expecting it.
Amy doesn't blame him for letting River kiss him; she's pretty hot, in an older-woman sort of way. Amy wouldn't go there herself – she'd never been with a woman, and she wasn't planning on it – but she could understand it; she knew a hot woman when she saw one.
For almost a minute they were kissing, and then they're not, they're just breathing heavily and either on the opposite sides of the room (likely) or staring into each other's eyes (unlikely).
But River Song is a criminal, and Amy's not that jealous of her life, even if she does get to kiss the Doctor occasionally.