I think usually when I see that idea, is more along the lines of "doth protest too much, methinks." A lot of the most vocal homophobes are at least slightly bi or at least seem to like either , partly just because of how common being at least slightly bi is and they go out of their way to deny it. That can be true and still much more casual homophobia by straight people can still be the most common source of homophobia and what pushes people to extremes in their denial. But I also wonder if the argument is sometimes made in hopes of making people afraid of being too homophobic in fear of being seen as gay.