I'm not sure that it's as simple as that your first pornographic experiences shape your sexual interests thereafter, as you imply, but I'll agree that there's a correlation. I just think that human memory is a fascinating and complex thing.
I can't remember the first "porn" I saw. But imagination is a wonderful thing; and I can remember "near-porn" things that I saw in my early teens that could be considered to be influences on my later sexual interests. I remember seeing some film, at about the age of 11 or 12 (with my parents, no less), in which a woman (riding cowgirl) tells her male partner that he's not to get off until she does, and thinking it was incredibly hot. In hindsight, it's easy to argue that this may have had an impact on my sexual interests.
But on the other hand, perhaps I remember that particular scene of that particular movie (whose name I've never been able to recall) specifically because it appeals to me now, as an adult. Memory is far more malleable than we are capable of understanding - or comfortable comprehending! There is no science in asking people what porn they first saw (i.e. first remember seeing) and how it affected them, because they're an enormous cognitive bias. Who's to say that we don't simply best-remember the sexual experiences that most-concisely fit with out current mental model. It's certainly the case with other areas, studies on memory show.
But whatever the case, I have a great attachment to giving my partners satisfaction as a priority. Contrary to the clip I remember, and in support of my memory hypothesis, this applies regardless of the gender of my partner. This is especially relevant as, at age 11 to 12, I was not remotely convinced of the possibility that I might be attracted to men as well as women.
In short; I'm not sure it's as simple as you'd like to think. Memory is a tricky beast, as my story shows.