If Spock had used that excuse it might have seemed logical. I suspect that it wasn’t used because the writers needed a stronger motivator for T’Pring and Spock to take a break and using that reason explicitly would weaken T’Pring’s decision.
I can understand T’Pring’s hurt feelings. She’s been nothing but supportive of Spock’s humanity and his search for identity. She tolerates the long separations. She desperately wants the relationship to work, gets to know his friends and family and she even gives him tips during the tea ceremony like a partner is supposed to do. To her, they’re a team, but to Spock, she’s still an outsider who needs to be compartmentalized in favor of his shipboard family. Spock claims it’s because he wanted to protect her and to an extent it’s probably true, but the bottom line is that he doesn’t trust her to be helpful, and that’s not the way a partner behaves.