I don't think that's the case. My parents have been married and in love for 24 years now, yet they are extremely homophobic.
There are other reasons why people are homophobic. Mostly it's religion (Which also bans a TON of other stuff, too, but since most people do that stuff, it was always "meant as a metaphor"). Other reasons are "it isn't natural" (Homosexuality is found in more than 250 species...), "it's an illness" (Which it isn't), "It's wrong and gross" (I find mucus gross, but you don't see me going around hating and beating people who blow their noses...), and the list goes on. There are a lot of reasons why people are homophobic, all of them stupid. But I think those all are just there to hide the real reason, the real reason being "I have a box on my head that says I should blindly hate gays, but that's stupid so I'll come up with some idiotic reasons since I'm too stubborn and stupid to take the box off".
Well... at least that's why I think people are homophobic.