Josh1992 saidI often wonder if it's because I have low testosterone. It could be that I'm a "late bloomer" but then again I don't want to feel like that and wait around when I could have a serious problem :
I doubt you have low testosterone at 19. Get morning wood, and spontaneous erections at other times of the day? Find it easy to get hard enough to masturbate, and when you do, is your ejaculate volume comparable to what you see in pornos? And do you shoot at least a few inches, rather than dribble? How often do you think about sex, and want to jerk off? Daily or at least several times a week?
Saying yes to most of these is not absolute clinical proof of a normal testosterone level. But it should give you some confidence, based on your sexual performance, that your level is not your problem. And if still in doubt, have a doctor's office do a blood draw for testosterone to confirm.
Also, 19 is far from being a late bloomer in the gay world, so I doubt that's a factor in your reaction, either. Rather, as I indicated earlier, I would look at psychological causes. Not that everyone must be the same, a range of responses is normal, and some guys in this thread have indicated their own dislike of semen under certain circumstances.
But your reaction seems more extreme, where you can't even watch it in a porno video. That could later interfere with gay sexual activities with other men. You might just try to watch more porn, and see if you can recondition yourself to eliminate your strong aversion.
As an aside, I might mention that when I was your age I sometimes got a very strong reaction to sexual imagery, but only after having cum. Like a switch being thrown, within seconds of an orgasm the same erotic material I had used to excite me now revolted me, and I'd quickly store it away out of sight.
A guilt response? I never did learn the answer, and by my twenties it no longer happened. Maybe related to what you're experiencing, something that will disappear over time? But just guessing from my own life, not an expert on this subject.