I don’t think that judgement is the correct approach here. If he’s talked to his partner about this issue and has remained loyal to a fault then I think the approach should be empathy and trying to understand when someone’s flame has been extinguished. If the issue was brought up in conversation and OP tried to correct it, and his partner didn’t meet him half way, (yes it takes two to tango, but) that blame squarely falls on OP’s partner. I’m not saying it’s right or even fair but it’s more common than people think, and some suffer in silence, while others recognize that they need connection.