Seeing the relationship growth between sang tae and moonyoung has actually been one of my favorite parts of the show. Their relationship began simply as a fan and an author and soon sang tae became leverage for moonyoung to get close to kang tae. I think the shift in their relationship began when sang tae moved in with moonyoung and she began to realize that she genuinely liked being around him, enough to even consider him a best friend.
Sang tae was no longer looking up at her, metaphorically speaking, but rather straight at her. He's no longer just a fan, he's her friend who he trusts hence the reason why he feels hurt and betrayed when he finds out about moonyoung and kang tae's trip. He wasn't just upset with kang tae, he was also upset that someone who he considered a best friend lied to him.
That is also part of the reason why sang tae doesn't want moonyoung and kang tae to form a relationship. Kang tae belongs to him, moonyoung is his best friend. He can't fathom to people he cherishes as separate entities to be together because he feels as though they belong to him, not each other.
