How can I convince my fiancée to abandon the idea of "friendship" with young men, which she describes as being within the bounds of Islamic law (permissible calls and meetings), especially since she considers those who oppose this idea to be "rigid and extremist"?

Women are among the greatest causes of fitnah (temptation/strife) for men, and the Prophet, peace and blessings be upon him, warned against this. Therefore, every door that leads to it, such as a man's friendship with a woman, which is a great door to fitnah, must be closed. The Sharia has commanded the blocking of means that lead to the unlawful, such as khalwa (private seclusion of a man and a non-mahram woman) and tabarruj (display of adornment). Friendship between the sexes is a slippery slope, even if both parties claim it is innocent. We advise the questioner to counsel his fiancée to abandon this friendship. If she complies, then alhamdulillah (praise be to Allah). Otherwise, he should refrain from marrying her and seek a woman of religious commitment, bearing in mind that a suitor is a stranger to his fiancée until the marriage contract is concluded.