Fixten
Just man up and add firia to the Mc's love life, it's not a harem if theirs only two. Besides you already put your self against the wall with the fact that female mc is open to it plus she dosent want her friend to be In a love less marriage. Also the mc revealed a lot of his trama to her already and that builds a lot of trust and feelings. Besides I think this works a lot better then most other multiple women stories, because you have fleshed them out and your mc had a anti harmen foil in the "hero" as an example of what not to be like
Hmm, if you said no harem at the beginning it'd be fine, and while some people might not read the story, I doubt most people would care, but to build Firia up as a character, especially with her pleasant mentality, obvious attraction towards the mc, and other forms of character development only to flat out deny any chance of forming a romantic relationship with her...I got to hand it to you author, you are a talented sadist.