1. Reaper is so blinded by the fact Geno doesn't hate him or that Geno can't die by him or something along those lines that he doesn't realize his feelings towards Geno aren't romantic.
2. Reaper is traumatized by the events that happened with Toriel that he denies his love to Geno
3. Likewise to #2, Geno is so scared to lose another person he loves that he denies his love for Reaper in fear. Or, Geno is to blinded by the need or want to save his AU and family that he doesn't realize his attraction to Reaper
You can change the names, character, story, anything. I'm just here to help and inspire!