Originally Posted by Miburo
No. He may be a closet fag, and only pretended to not like the guy on guy one because he wanted to give off the appearance of not being gay. And, like someone else pointed out, it was also a guy on guy image with the guy he's been lusting after for the last couple of years. So him being upset to see that guy with another guy is also another possibility.
Notice how Kakashi wasn't all "OMG EW GROSS YUCK LOOK EVERYONE I DON'T LIKE GAYS" when he saw it. Because Kakashi is comfortable with his sexuality and doesn't feel the need to try to make sure everyone around him knows he's not into that shit. Kakashi even makes a homoerotic joke with naruto later on, playing on Naruto's obvious insecurities regarding his sexual preference, if I recall correctly.
You do recall correctly, since Kakashi gave said joke around the time he was teaching Naruto Chakra-stuff with Yamato.