Again, what Jason did was awful, but it was done without him pausing to consider the consequences. His rage blinded him to the probable outcome until it was too late. And as far as Thomas and Shane, there will be further details about the relationship, so I don't want to do too many spoilers about how Thomas really felt, other than say he did pursue Shane.
However, I can explain why Shane falls for Thomas and doesn't try to have a relationship with Jason.
Just think....the kiss happens and Shane is drunk. Not too drunk to remember, no matter what he said, but definitely drunk enough to be confused as to the context and how much it means to Jason. Unsure and scared, he runs. And then what happens....Jason willingly accepts Shane's not remembering and proceeds to act like nothing happened, and then basically ignores Shane for the rest of the summer. A summer spent with his hyper religious stepmom who dislikes him and a father who has moved on to his new family. They have also become deeply involved in the stepmom's homophobic church. So Shane decides ignoring the kiss and his feelings for Jason is the best option.
But it has stirred up his awareness of his sexuality, and he is very lonely (especially with a distant friend) and desperate to be loved, not to mention completely inexperienced at relationships. So when a good looking, charming boy shows up who proceeds to deliberately woo him, how could he resist?