Hey Trasher
i get what you mean, but most things can be explained.
why connect they so fast? because they were really close 4 years ago and Ava has nowhere to go. Even when the 4 years are a big part of Avas life, it doesn't means she has forgotten everything, about Byron.
why ava wants to experience sex is answered later in the game. She gets flashbacks while teasing around with Byron, what shows, that there are still 'scars' under the surface. She is forcing herself a bit... I can't answer how realistic that is, but I heard about weirder stuff.
Why Dallas and Byron are close can be answered by Byron's and Dallas' relationship to Ava. Dallas is Avas best friend, so with no doubt they were often together. The game explains that Byron was often at Sharols (Avas mother) to spend time with Ava, so he knows Dallas of course.
As i said, the answer for that will follow ^^
In my opinion the story is plausible. Humans aren't always logical and so some choices of the characters can be explained by that fact