he says he is straight but his dick is hard when I finger his hole

After a nice shower at my house i asked if he was willing to act as a submissive for my pleasure. he agreed, but said ''i never done anything like this before. i'm straight.''  i answered ''sure, buddy. don't worry. i know how to handle this.''  his dick was hard long before i ever touched him. don't know if was because of the handcuffs, the blindfold or the feeling of being dominated, but something made this guy hard as a rock