This phrase is a way to make fun of people who say 'that's so gay' by changing it to 'that's so straight'. Many gay people (or people who think its offensive) use it instead of 'That's so gay' to show that its a hurtful phrase.
Amy: I got a ton of bio homework to do tonight, we can't go on our date...
Dane: Damn, that's so gay!
Amy: That isn't gay. That's so straight! (like you lol)
Dane: *crys in corner*