Well a friendship is fine, as it is fine when a grandfather befriends a foster child or grandchild, which is what this would be, however I think some are fooled by Toushiro's appearance and think this would be a typical school-chum kind of relationship. When you say "romantic friendship", I think we're leaning towards french-kissing and touchy-feely things, which leads us back to the grossness of their age difference.