I didn't understand that just now, someone explain.
Honobe's using innuendo to suggest them having sex. She starts out with,
"Like how fireworks shoot up, bang! And it's over." When Kusano says he wanted to have something like that. Honobe adds,
"Ok, wanna try?" before sitting on the desk facing him and adding,
"Watch the fireworks disappear into the sky, with a bang?"
Kusano, surprisingly, understands what she's suggesting, but isn't interested in her at all, so he destroys the metaphor by pointing out that
"Wet fireworks won't shoot up." Honobe gives it one last try, suggesting
"Dry, then?," (aka, sex without foreplay) but he shoots her down again by saying dry fireworks sound dangerous.