Porn aside, I can think of three reasons that play/have played a part in my not being gentle enough that I imagine have influenced other guys:
1. Men's genitals work very differently: Penises respond to/need a rough kind of treatment, and mostly we need that sort of intense, "manhandling" stimulation to get off. Successful handjobs (male version) are not delicate matters. With that as my model in mind, I analogized that to the clit thinking that's what also works for a woman. Even after learning that manual clit stimulation works best, most often, with the more gentle/delicate touch, when doing it that way with a woman, I tend to match her enthusiasm for what's going on--that is, as she gets more turned on and into it, I get more turned on and am more into it, and as she's more excited, so am I and I get more energetic, lose myself in the moment, and start working to push her over the edge. So, I can get overzealous because I'm responding in a primal, more emotive and non-thinking way--which is cool psychologically and emotionally for both of us because we feel so connected, and as she nears orgasm getting very focused on the outcome rather than the process (yes major pun). Despite knowing this, it's still a problem sometimes as I want to and do participate emotionally and get caught up in the moment so much and lose sight of what I'm doing and what she needs/wants. It can be difficult sometimes to balance the intellectual and emotional in the moment and give her what she needs/wants, but I'm aware of this problem and talk to my women about it to find ways to stay in the moment but not get over-zealous.
2. It's not easy to observe what really goes on with a woman self-stimulating (true way, not porn way) in photos/videos because the perspective of the lens isn't as easily and usually up-close enough to show specific details of skin on skin. With handjobs, it's easier to capture the process in images or to infer what's going on even if it's not directly shown. E.g., seeing handjobs in mainstream movie where they just show the woman's arm moving up and down vs. trying to interpret seeing a woman being manually stimulated--which isn't depicted as much in film, etc.
3. When I've seen girlfriends masturbate--sometimes after asking them so I can see what works for them, and watching women doing true, non-porn-style masturbating, they seem to rub themselves very fast and rough--at least when they're closing in on orgasm. That was misleading at first--and maybe still influences me despite knowing better now--when I didn't know they were really rubbing like that on the skin just above the clit rather than on the clit itself.
I really want a woman to explain to me what works for her--and to show me, esp. how to bring her to orgasm using my hand with her clit. I think guys in general need to have it explained given the reasons making it easy for us to misunderstand how it works for most women.