Historically, dating involved a man spending money on a woman, nominally for purposes of courtship to prove he was a good provider, but often simply in hopes of sex in the immediate future. As social norms have changed -- such that casual relationships and sex outside of marriage have become more acceptable while women are more able to establish an earned income -- we still tend to default to men paying for dates in hopes of getting laid.
So dating has something of a tendency to be a polite form of prostitution where men spend money in a gamble in hopes it leads to sex. If men can legally spend money and state up front they are expecting X, Y or Z and get it, why "date"?
Though when the OP says they are dating an online sex worker, I assume they mean they are in a relationship to this person. The word "dating" gets used rather sloppily to mean a variety of things -- which isn't all bad, mind you, but sometimes interferes with clear communication.