Question aimed more at men than women. What do you men think about when you watch porn? I ask because how can you be considered straight if you find another naked man having sex with a woman arousing? Do you pretend the man isn't there? You see the man's penis, so how can you be straight if you masturbate to this?
Also, why do people find hearing other people have sex arousing? What are you thoughts when you hear another couple have sex?
Is it not easier to imagine yourself as a human being of the same sexual orientation as yourself than an inanimate object??
The inanimate object is the same shape shape as a penis and even looks like a penis, so you can easily imagine it as being your penis. When you imagine being the guy, there is a strong chance that a presumably straight man will think of gay thoughts, as sexuality is not as polarized as people think. A previously straight man may become a bisexual male when watching porn.
The man you're imaging is you will likely have a large penis, whereas you won't. So as he pleasures the woman, you're imagining that his penis is your penis and by doing so, you recognize that his penis passes some kind of aesthetic judgement. If it doesn't, you're likely going to find the video less physically satisfying, as the woman won't appear to be sexually satisfied because the man's penis is smaller. So you're not going to imagine you're the man with the small penis, and your recognition of this, means you are able to tell apart, and value, male penises, which is inherently homosexual.