Why did Michael Scott have a crush on Ryan in The Office (US) even when they both were straight?

Yeah, it's called a 'man-crush' or 'bromance.' It is nonsexual but more than just basic friendship. If it is shared, it can turn into a 'best friend' scenario that becomes as close as family. Often it is just a passing admiration or fixation, however. 

It was a man crush, a one-sided bromance. There are crushes where there's sexual attraction involved. There are also platonic crushes where you're just really into somebody and can't get enough of them and just totally want to be best friends and are thirsty for them but don't want to have sex because it's a purely emotional and psychological attraction, not sexual.