To tell the truth, the Doctor didn't want to read Missy's journal, afraid of what he'd find inside. But now that the Master was inviting him, he knew he'd have to, because eventually he'd ask for his thoughts and opinion.
"Isn't it obvious from the company I keep?" The Doctor, continued to hold the Master's hand when he told him, "I'm may be bisexual, but I have a leaning toward females." He knew this was going to upset the Master, so he added, "You asked, so I told you."