Actually, Xiao Tian wanted to take a picture of Liu Ning with his sperm in her mouth, so that he would be able to see it whenever he wanted.
However, he suddenly changed his mind because there was a possibility that other people would see it, and it would be a problem for him in the future.
Liu Ning was his woman, so if he wanted to see her mouth filled with his sperm, he could just do it, and he was sure that she would grant his wish.
"Ning'er, drink it." because his sperm was still in her mouth, Xiao Tian knew that she was wavering whether she should drink his sperm or not.
Liu Ning was different from Shi Fei because she was still not used to drinking his sperm. If it was Shi Fei, Xiao Tian believed that he didn't need to say a word, and she would drink his sperm without thinking twice.