>>528256629
Stupid people that don't understand art always get upset when the protagonist they self insert as doesn't "get the girl", or whatever the equivalent of that might be in any different scenario. They see all art as something that should cater specifically to their needs and desires, and automatically view properties that don't strive to do so as inherently flawed.
Alphys and Undyne weren't introduced as endearing or romantically desirable, so them getting snatched away in a lesbian twist wasn't seen as a betrayal of that viewpoint.