Borat asking the driving instructor to be his boyfriend

Um... dude. The only real point of Borat was to freak people out by being weird as possible. It was unexpected of him to ask the driver "will you be my boyfriend", that's why he did it. He was just trying to add in a random bit of unpredictability to the character, one that popped into his head. It was a good target to do it since that instructor looked definitely non-gay.

By the way someone can be "straight" and homophobic but still turn gay on you. Like, the joke was supposed to be suggesting Borat has some latent homosexual tendencies that are coming out because he's no longer in his backwards home country. Or something.

Or maybe he's just a goof who doesn't understand the meaning of the word boyfriend.

The driver seemed fairly relaxed about it so he obviously wasn't scared that Borat was gonna somehow start seriously trying to get some action out of him or anything. He thought the guy was just a goof, which is what the general idea was.

I found it pretty funny cause of how rediculously silly it was.

it's character inconsistency but the reaction is funny. It wasn't a macho-like reaction.

It was the best scene in the film.