Relationships when the evolved realm was involved were indeed complex, but each situation was unique.
In most cases, the evolved side would lose interest in mortal matters due to its vastly different perspective. It was like asking a dragon to understand the concerns of an ant. The two saw the world so differently that they couldn't possibly find common ground.
Khan had faced that problem with Monica, and, despite never losing interest and his best efforts to remain involved with her way of life, he simply couldn't. Her very life had begun to appear so inconsequential that he struggled to see any value in it.
That culminated in the issue with Princess Rachel. Khan knew a relationship with her had always been impossible, but he had been willing to consider it for political reasons.
Khan's concept of love completely conflicted with that, but his standards had become so high that he had been ready to give up on meeting them altogether.
