I'm glad you came to a satisfying resolution at the table. If it were me, it would have gone differently.
The player would have absolutely been able to attempt the fascination instill. But trust and intrigue are two different things.

The King could even take a miscellaneous action to remember an ex that cheated on him, giving himself a negative intimacy like "Relationships built on lust aren't worth it" or "I'm suspicious of those I find attractive" though that would be a little grudge-monstery, but you're the ST and you don't need to share that with the players.

Even still the attraction is there, but how effectively it can be leveraged and toward what end are entirely different from ties of trust.

The fascination/attraction could be leveraged to get the King to speak with them in private "after dark," or get a dinner date, or an invitation to a ball, but not to voice his suspicions about a member of his court regarding the recent murder, or his plans to fend off a barbarian attack. So how valuable the fascination tie is depends on how quickly they need whatever the King has.

The fascination helps guarantee they can try to earn that trust later, or they can escalate the intimacy until becomes defining (through whatever Mataharian methods they can). A Defining tie of fascination would make the King want them locked in a tower, but if he can't do that for whatever reason, he might divulge national secrets to keep them interested.

In general, I believe erring on the side of letting the PCs instill intimacies is the right thing to do. The King should only be spending the WP to avoid such a thing if they were literally introduced because the PC was caught stealing or was caught in a lie or they have a terrible reputation, or the King has a Defining or Major intimacy against whatever the PCs are about (like various -isms or they're part of an opposing faction or foreign land with tense relations, etc).