I don't see any reason why it should be.
Pleasure is not illogical.
Pleasure is irrelevant, is what a Vulcan would almost certainly say to that, especially physical pleasure.
I think
This Side of Paradise shows that Spock, when he's in his right mind, was not into casual romance stuff, much less casual sex. It took mind controlling spores to make him react to anything similar to that. I don't believe for a second that Prime Universe Spock was going to be having casual sex, certainly not with T'Pring pre-Amok Time. Maybe if he had a long term relationship with someone or another Amok Time incident I'd believe it, but I don't believe that Vulcan's in general are nearly as
Unless they try to retcon T'Pring as some long lost love interst, instead of just a woman he was promised to as a kid and then never really interacted with after that, it just doesn't make sense for him. That seems to be the retcon they're going with, but I don't accept it. Its only a small subplot, I loved the episode besides that, but it is 100% against what the character would do, and definitely something that only happens by ignoring/retconning Spock's character.
The lore has never said they don't. DC Fontana has said in the past that they do.
Besides T'Pring was never very logical, re-watch Amok Time.
DC Fontana was certainly not an authority on Spock, I don't care if she wrote the (extremely overrated) Spock backstory episode of TAS. Spock in the Prime Universe was never into that kind of stuff, based on how he reacted to romantic scenarios. He didn't even have interest in T'Pring, who at the time he had obviously not seen since they were much younger, outside of the Amok time situation.
As for T'Pring, in
Amok Time she was acting in a way she thought was logical. Logic doesn't have to be kind or lead to only good things. To her, what was important was that she had no interest in Spock, she wanted to be with Stonn. But Vulcan, for all its talk of logic, is a fairly illogical culture, with a bunch of mysticism and frankly messed up traditions that don't fit a supposedly enlightened society (which Enterprise of all shows even called out). So she couldn't just say no to being with Spock, at least not by the standards of her culture (although I bet Federation law protects citizens from things like arranged marriages if the person really doesn't want to be involved). To save face, she used an archaic Vulcan tradition to try to get what she wanted. It was cruel, and potentially murderous, but it was 100% logical from her perspective. Like she says in the episode, If Kirk killed Spock he wouldn't want T'Pring, and Kirk obviously didn't want her.
Everything she did made sense, if you understand that she wanted to get her own way without going against traditions then she was being perfectly logical. She was also a horrible person, she could have just chosen Stonn at any point and broken up with Spock, but presumably that was looked down on by Vulcan society and she cared more about that then she did people's lives, which makes her a bad person but not illogical.
