I don't think he "wanted" any thing with wren. what he wanted with wren was superficical and the b*ner he got was a quick physical reaction to wren throwing himself onto tristan .Tristan even felt bad that the reaction happened even tho he couldn't control it and thought of ethan while...yk doing it
Holy bad reading comprehension, Batman.
Wren was a father figure to Tristan. He genuinely loves him. And yes, at one point he was interested in Wren romantically. But he outgrew that. I'm sure that his familial feelings for Wren and the memories of his romantic love for Wren made it confusing for Tristan and hard for him to push Wren away. But the reason he is crying is because he knows that Wren's actions will change the dynamics of their relationship in a bad way.
Tristan liked it thou that's he was crying because he felt guilty because deep down he wanted it