
The mere fact that Aiji himself wouldn't want Towa to leave is the answer here. As you can see both Aiji and Seiji loves Towa because he's the only one that tried to understand them. Aiji first thought that if he leave, Seiji can start a new life with Towa but Towa never allowed that and still searched for Aiji even after all those things Aiji did. That itself made Aiji love Towa. Now both of them wouldn't want Towa to belong to another person. Both of them would only want Towa for themselves. Its a three way relation ship where in the three of them loves each other plus Aiji himself wants Seiji to mark Towa so he could be assured that Towa will belong to only them.
I feel sorry for Aiji.why I am so bothered? I like Towa but there is no place for him.Please find someone for you......Damn I need time to clear my head. Ya its just a story,story,story...