Some married men (and couples) think that it is useful to read sex manuals or watch pornography because it is part of their “sex education” — they will be better at their lovemaking if they watch how others engage in sexual activity. I think that is (in general) a mistake and that their own trial and error is better.

I obviously can’t deny that, especially earlier in a marriage, you could learn some things by watching videos of other people having sex. It is certainly possible that you haven’t thought of doing things in a certain way, which you might pick up from watching such videos or reading such books. So why not do it?
First, and most importantly, marital sex is intensely personal. It’s not just doing the same thing other people do. We are unique individuals. We have our own preferences, and these are not identical. A husband’s preferences and a wife’s preferences are, to some extent, different. And one couple’s preferences are different from others’, too.
Second, there is a genuine delight in discovering what “works best” by simply trying different things and seeing how you (both husband and wife) like them. “Trial and error” – the process of trying things and discovering what doesn’t work, and (more importantly) what does work – over the course of a lifelong marriage is great. As I have said elsewhere on this website, I think the general pattern in a marriage is to find ways of making love (which include some variety) that are really enjoyable for a while (months or years) and then to make some changes in your lovemaking when you find (sometimes “stumble across”!) something that really works well and makes you both even more satisfied.
To give an example that combines these first two points (both the uniqueness and the trial and error): in one marriage, a wife didn’t particularly like her husband (who was inclined to “get right to it”!) to be aggressively massaging her vulva, especially when they were starting their lovemaking. She wanted to start more slowly. But she also needed some stimulation to get more into it. They discovered that, if her husband put his knee up against her vulva (firmly but gently) as they kissed and did breast play, that helped to arouse her without the feeling of being “pawed at” too much. (Actually, they discovered this after he shifted from using his hand to massaging his forearm gently against her vulva, which worked better, and then shifted to using his knee.) That became a standard part of their lovemaking for many years. [Imagine their surprise when, after doing this for three or four decades, the “knee thing” very recently became a viral TikTok meme – the latest “innovation” in foreplay!]
Third, there are “downsides” to learning about sex by watching others do it online or in a book. When you find something that you really like, what happens if your wife doesn’t like it? You have, in a sense, gotten your hopes and expectations up, but now the result can be frustration because she doesn’t particularly want to do it. For example, you can find SO many “sex positions” online, and some of them may really appeal to you — but it can turn out that they don’t appeal to your wife at all. That can create tension between you. Why not just talk about what you’d like to do and try it?
And if you find something you like being done in a video with a very good-looking woman, that video – that woman – may “stick” with you. Do you really want to be making love to your wife and thinking of another woman? Is that going to help you really make love to HER?
I’m not saying that you should never look to find more information about marital sexuality. (How can someone who is writing on a website called “maritalsex.org” say that?! Though this website is focused especially on how to keep marital sex personal, and not a matter of technique.) But I would say that it’s important to avoid pornography – which is not about love at all – and that, for the most part, the best way for couples to learn how to have better sex is by trying things on their own.