Back in the day- porn was pictures of nekkid girls.
Then it became pictures of nekkid girls having sex with average or even sub-par looking guys.
Then it became pictures of nekkid girls having sex with guys who were well hung, thou average looking... and then girls having sex with guys who were hung AND well built.
Thru the whole evolution of porn, the imagery has come to focus more and more around the central image of the erect, veiny cock and the cum shooting out of it.
Today--- if it doesn't feature beautiful close ups of gorgeously engorged, cum drizzling cocks, its considered 'soft core' porn... i.e. "not really" porn at all.
The result has been to turn all males who indulge in porn into cock voyeurs, who are being conditioned to become sexually excited at the sight of a beautiful cock.
More and more, sexual stimulation is not merely predicated upon the nude female... but includes the imagery of OTHER men's cocks.
More and more, this conflates in the male mind their source of arousal.
Since, (as I believe) all human males are wired to enjoy sex for sex's sake as separate from how sex plays into love...
This continual bisexualization of sexually related imagery can not help but lead many males to thoughts of sex with one of those long, thick rigid cocks that factor so prominently in every vicarious sexual experience we call porn.
I think what you are feeling is perfectly natural, given today's culture.
I think the poster's on this forum make clear that bisexual fantasies and activities are far more common than most people realize.
I think, as a culture, men are becoming MORE likely to be bisexual as a result of greater social tolerance, and exposure to modern porn.