How is he a coward for marrying others, when he is the emperor? That position comes with responsibilities, he had to fulfil those. It's realistic considering the times. I think we are so used to the stories whereas the Emperor behaves like there are no rules and stuff to follow, forgetting that such things rarely occurred.
If you think about Poli (how she feels until now in both the novel and manhwa), she doesn't see him romantically. Something we should be happy about is how concerned and actually supportive Lucius was. He knew that confessing would not get Poli to like him, she doesn't even imagine it. He did not force her to become an item with him disregarding every effort Poli used to get to where she was.
So give him slack.
So far in the novel, I do have my good and bad opinions regarding him, but thinking about how much he worked to unite the continent and how long (10 years) he fought alongside Poli that he knew that things would take time to flourish between them. He couldn't just wait for that when he has stuff to do, like leaving an heir and such.
And Aino isn't better, spoiler (this one was harassing the girl he liked) at least Lucius is better than that.
HOWEVER, I do not recommend u to drop this story. If u like tough woman and pretty forgiving to ML who messed up a lot, then u should continue reading it. I dunno how similar the manhwa will be to the novel, but we know the manhwa made Lucius a better person than the novel counterpart. However, (spoiler ahead)
I don’t like how he’s gonna have 3 wives and a daughter from one of the wives. And 2 of his wives died, and the third wife asking for divorce. Like I know I know he doesn’t love them but I feel sorry for the wives and also MC. Like hmm idk I really don’t like this kind of ML who is a coward, like Aino says.