Well here we disagree, to me the fact that he has feelings for her, even if he can't act for obvious reasons, still qualifies ad romance.
And looking carefully at the story so far, we are clearly seeing that Yamato is slowly filling the void in Yakumo's heart due to the death of her husband. Given the premise of the series, to me this is enough and I'll be happy if in the end we'll have a grown up Yamato going out or even be married to Yakumo, but if One is in search of a story focused in romance this isn't clearly the right manga.