Can We Resolve Our Culture Clash?
When he tells you that he "cannot cope" with the length of your skirt, he is also telling you that, although he says he isn't a "strict" Muslim, he has taken to heart some of the values and customs of his upbringing. His behavior is a natural ‑- even an inevitable ‑- consequence of his exposure to certain beliefs and traditions. And it's also natural for him to have a strong reaction when he encounters something very different from what he knows.
You, too, are a product of your upbringing, so you cannot see anything wrong with wearing a knee-length skirt. But once you marry him, you will see your behavior not only through your own eyes, but also through his eyes and the eyes of the people in his community.
Your fiancé says that he does not want anyone from his community to think or talk negatively about you. This shows that he is still very involved with his community and cares a great deal about what they think and say. Once again, this is not a bad thing, but it can become problematic if he marries someone who does not feel and think the same way. To avoid upsetting him, you will have to alter your behavior to suit the customs of his community. Are you willing to do that? Do you know what this might involve?
Consider that these differences in culture are likely to become even more of an issue when children come into the picture. We want our children to have experiences similar to the ones we've had. We want them to share in our culture, religion and beliefs. But when two parents come from very different cultures, religions or points of view, this issue can create serious conflict.
You and your fiancé must realize ‑- and be able to speak openly about ‑- the larger implications of this issue. If you do wear shorter skirts (or adhere to customs or dress codes different than those of your fiancé), he is likely to feel bothered. You may also become frustrated that you are being asked to give up what feels natural to you. These feelings are already evidenced in your question.
I would take a long look at all the differences between you and your fiancé and be sure you understand and are willing to accept the sacrifices that your marriage might entail.