If it's not one of those "When they were little." scenarios, I'd more assume that it was Zuikaku kissing her, more than anything attempted by Shoukaku herself.
Also, I love the way this artist draws Souryuu. He constantly portrays the most adorable despair with her.
Y...Well, even Hiryuu and I still haven't done it before!Huh?*sigh*Have you kissed her yet?Souryuu... As expected, it's too early for kissing...You're right.-Woma...By the way-...Huh!?Because Shoukaku looks like a prudent-