That's not true. Few humans are 100% straight. Sexuality is very complex and there's no doubt that plenty of homophobes are repressing desires, or even fleeting fantasies they have had that they just don't know how to handle. It's anecdotal but for me the only men I ever knew who hated gays were not at all comfortable in anything other than very strict gender roles, and were very over-concerned about their manhood and how it is perceived. Men I've known who are totally comfortable around gay men are extremely sexually secure. It's something I notice because it's extremely attractive.
How about for Lesbians?
Work the same way?
I must be a lesbian because I only like sex with women?