沒錯, 是子句.
What you guys don't understand is (that), for us, kissing is as important as any part of it.
這裡的 kissing 是動名詞.
其他例句:
-What he didn't know was that, staying silent is more powerful than speaking out loud.
-What we couldn't see was that, for them, walking away was more helpful than standing there watching.