Renly drinks from a goblet. His bride Margaery enters. He warns her that he's had quite a bit of wine, so he's established his reason for the weird way he's about to start acting. It's stilted and awkward. He compliments her gown, which seems a trifle stereotypical. But really he just wants to talk about anything other than the topic at hand. She reveals her breasts, which are perfectly relevant in this scene. I know that it's fun to complain about the gratuitous sex scenes in this show. I've done it myself and I'll do it again. Because it's fun! But this scene is about the sex itself and the breasts are important to the characters. What's actually important is that Renly is actively uninterested in the breasts. Or possibly disinterested. Whichever. He keeps talking while she kisses him. She grabs his dick and he blames the wine for his complete lack of response. So she gets to work. That's a euphemism. He closes his eyes and concentrates. Then he apologizes. She asks, "Do you want my brother to come in and help?" He is speechless. She offers to have Loras come in and get Renly started. "He wouldn't mind. Or I can turn over and you can pretend I'm him." She's great! This has to be one of the most understanding sham marriages ever. She tells him to save his lies for court because the best way to stop their enemies from tearing them apart is to put his baby in her belly. "With me. With me and Loras. However else you like. Whatever you need to do. You are a king." He's lucky! He looks super-uncomfortable about this whole thing, but he seems to have landed a wife who not only doesn't object to his sexual orientation, she doesn't mind having a threeway with her own brother. Although I guess incest isn't much of a dealbreaker in this world. Anyway, my point is that Margaery is awesome.
Back to King's Landing and the explanation for Tyrion's Obvious Plan. Cersei is yelling at Tyrion for planning to marry off her daughter without telling her. Eventually she gets into specifics and it turns out she heard the story about Dorne. So her spy is Pycelle, then! She also has specific objections to the plan, because the Martells loathe them. Myrcella will be a hostage, which Tyrion prefers to rephrase as "guest." The difference, I think, is that Sansa is a "guest." When he was being raised by the Starks, Theon was a "guest." Jaime Lannister is an actual hostage. Cersei is not actually interested in the semantics of hostages. She'd much rather threaten Tyrion. She says he won't be safe just because he has a piece of paper saying he's the Hand. Ned Stark had one of those and look what it got him. Tyrion tells her it's already done. And also, there's going to be at least one army attacking King's Landing pretty soon, so even if there isn't a peasant uprising, getting Myrcella out of the city is the best thing for her. Tyrion gets explicit: "Do you want to see her raped, butchered like the Targaryen children?" She pushes him and he leaves.