Yukimura had a tendency of being affectioate at the worst of time. By the worst of time, he ment whenever Kojuro was watching.
It was no secret that the older man raised the dragon, from an egg to the full scale creater he has today. He considered Masamune to be his son. Therefore, once Masamune hit puberty he was very carfule in explain that he would one day have to marry in order to make heirs.
Kojuro planned on picking out a bride he knew would fit Masamune, but he soon learned that Masamune had no intrest in girls. That was when he made sure that nothing with male genitila was left with Masamune.
He made a mistake with Yukimura. The tiger cub seemed harmless enough. He even looked as if he would be a good influence on the younger.
Masamune relied on that ignorance. It was how he was able to get alone time with Yukimura, even after he told Kojuro that he was seeing the other male. It gave him the freedom to kiss and hug without prying eyes. They did things that would make people blush, all with Kojuro being none the wiser.
Now, Yukimura wasn't called a Tiger for nothing. He liked to paw and smother Masamune when he got the chance. It's why Masamune prefered being alone with him. Today, however, Kojuro decided to stick around longer than he normaly did.
It left Yukimura anxiouse to be closer to him. Now he just couldn't take it any longer. He lunged at Masamune attaching himself to the dragons neck kissing and sucking.
Kojuro fainted.