There was never any question in my mind about Minato breaking the S-plan. It's just a matter of when and how.
Tsukiumi reaction to Seo kissing the Thunder Twins was like she was witnessing a live orgie. Granted, her feelings for Minato would make her do things that would be 'out of the ordinary for her' for his sake. But considering how conservative she is, I don't think open displays of affection are as easy to her as it would be for Kazehana, Musubi, and Matsu. Even if it's with Minato. Though my opinon of this will change as the series and her development continues. I do want to see her norito though.
As for Homura, I don't think PDA is a problem, so much as who it's with. He/She still has the mind set of guy, and as he once told Minato, the fact that his ashikabi is also male when he was expecting a female is something he/she didn't expect. I mean, the man was basicly an escort and had tons of girls all the time. To go from that, to his/her current situation is something he won't jump right into I think. To be honest, I personally think Homura will decide to prolly become full female and that might help his situation a little. But I digress.