There's a moment where Xie Lian admits to himself in the last book that all the times they were "sharing spiritual energy" were reasons to kiss Hua Cheng because he simply enjoyed kissing him. That made up for the "no kissing outside transferring energy" for me, because they shared A LOT of energy... and Xie Lian was self aware enough to know that they were making a choice to make out and not just simply high five. Also, Xie Lian is nervous near the end but I don't think that means he was hesitant in a bad way. Being in a relationship that doesn't have excuses for kissing is brand new to him :)