Whatever impression fanfiction may have given you, Darcy is never refered as a man of the world in the book. Also, I don't think this expression aludes directly or solely to sexual behavior; a man of the world is someone who has a certain level of expierience of society and affairs and some sophistication, which in turn probably meant that he also has some expierience with the opposite sex and is comfortable interacting with them. However, that doesn't neccesarily mean sexual relations: Elizabeth, for instance, seems very poised and in control in her interactions with the men she meets, potential suitors or not, but she was certainly a virgin.
As for Darcy's probable sexual history, really we have no way of telling. The conventional morality of P&P's era was that there was nothing wrong with men having premarital sex, provided they followed rules and kept to certain standards, e.g. not seduce innocents or married women, not force themselves upon unwilling partners, be discreet, etc. Darcy was an honorable, serious-minded and responsible man, so I think any relationship he may have formed would have been conducted on these lines. On the other hand, he was not always the man we see in the book; I don't think it's out of the question that an 18-year-old Darcy might have visited an exclusive gentleman's club. My own opinion is that he was probably not a virgin, unless he had really strong religious beliefs that are not apparent in the book. But I think he was probably largely celibate, considering that for an honorable man, there weren't really all that many opportunities to have sexual relations outside of marriage.
As for Darcy's probable sexual history, really we have no way of telling. The conventional morality of P&P's era was that there was nothing wrong with men having premarital sex, provided they followed rules and kept to certain standards, e.g. not seduce innocents or married women, not force themselves upon unwilling partners, be discreet, etc. Darcy was an honorable, serious-minded and responsible man, so I think any relationship he may have formed would have been conducted on these lines. On the other hand, he was not always the man we see in the book; I don't think it's out of the question that an 18-year-old Darcy might have visited an exclusive gentleman's club. My own opinion is that he was probably not a virgin, unless he had really strong religious beliefs that are not apparent in the book. But I think he was probably largely celibate, considering that for an honorable man, there weren't really all that many opportunities to have sexual relations outside of marriage.