If a straight guy can turn gay can a gay guy turn straight?

No one explicitly "turns gay". What happens is that society pushes a heteronormative narrative. Everyone is assumed to be straight, and in fact the word "gay" is used externally of its meaning of "homosexual person" as a general insult for the purpose of being insulting...because society had believed (and most places even today still believe) that being "gay" is one of the worst things someone could be.

When someone experiences same-sex attraction, they thus realize they aren't entirely "straight". Generally, this also sees a retroactive understanding of their past relationships and expressions and interests. Thus the evidence that they are gay has always been there, but no one paid attention to it because no one considered the possibility that they could actually be gay. And if it weren't for society (whether parents or friends, superiors or peers or the law itself) mandating heterosexuality, then they and others would have been able to freely notice the clues really early on in their life.

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A movie that expresses this conceit is "But I'm a Cheerleader". The girl is a cheerleader dating one of the school's football stars, when her parents get the idea that she's a lesbian. She ends up at a "Pray the Gay Away" 'summer camp' where, after protesting for most of the second arc that she's a "normal girl" and "isn't a lesbian", she falls in love with one of her fellow camp-mates and realizes that, yeah, she's actually been a lesbian this whole time and only thought she was straight because she didn't know there was anything else.

No, you can't "turn gay" or "turn straight".  You are what you are.

No-one 'turns' gay or straight.

It's the way you're born.

Straight guys do not turn gay.  Gay guys do not turn straight.  A person's sexual orientation is set from the time they're born.  All that changes is the individual's realization of it, self acceptance, and feeling of freedom to be out and open about it, all depending on the environment they live in.  Your premise is false.

Straight men only turn gay when they discover the joy of gay sex.

And since there is no joy in straight sex, gay men would never turn straight.  

There are people that have fluid sexualities - but they are mainly women. And they keep going back and forth to lesbian and straight anyway - it’s not a permanent conversion. With men - it’s mainly fixed at birth. You are either gay straight or bi. A straight guy that experiments with a guy just won’t like it and will stick with women, likewise many gay men try it with women because they grow up thinking they’ll be straight - but again - it’s just not for them. They can even a marry a women to conform to social norms - but when she’s not looking he’ll seek out a guy on the side. Some straight men are gay porn actors or prostitutes to gay men - but it’s just a cash transaction - in the their off time they stick with women. 

The straight guy you are referencing was never likely straight. Just acting. 

anything is possible