Masuo will refuse to have sex with anyone else but Reiko and Aoi.
After seeing all heart break events Masuo will go in coma and there you will be able to select the path. Get previous ring back and get a new ring that is more corrupted.
I also forgot to mention Masuo can obtain another ring during normal gameplay. This one can't be worn since it can't be removed from the severed finger. It will be available as an equipment. As long as you don't make right choice in an questline or don't initiate that, the other ring will stay in his possession.
And before you ask purpose of that ring. You don't need to 100% Dahlia's ring effect. More options will be available at the cost of loved one refusing sex with you.