When Alan finally let Erik enter his life, he thought it wouldn't work out and that he would eventually get tiring of him. However, the truth was that instead of becoming tired of Erik, he had completely fallen in love with him. Alan found himself wanting Erik more and more, and his love grew deeper day by day.
He had promised Erik yesterday that he would leave him alone tonight, but what Kevin had told him—or rather, implied about the fact that Erik had developed his combat intelligence skill because he wanted to protect him more than anything—had turned him on to such an extent that he was struggling to manage his own emotions. All he knew was that he wanted Erik badly, and luckily for him, Erik was offering himself without any resistance.