Why did Freddie become gay?

There's a scene where Freddie is touring the US, and he calls his girlfriend from a public payphone, he tells her how much he misses her, and then he sees a trucker entering a public toilet and his eyes immediately sparkle and just like that, bam! he becomes gay...
He didn't show any interest in guys until then, how did he become gay by just seeing a truck driver, it doesn't make any sense....