Dirty work = sex, specifically sex that results in her orgasm.
The song's about a guy who is the man she cheats on her husband with (when he's out of town, she lights a candle for romance, sends the maid home early), and the moral dilemma - sex or cuckold the hubby. He's unable to overcome his desire for her, despite the terrible risk, every time - thousands of times before.
Dirty work = sex, specifically sex that results in her orgasm.
The song's about a guy who is the man she cheats on her husband with (when he's out of town, she lights a candle for romance, sends the maid home early), and the moral dilemma - sex or cuckold the hubby. He's unable to overcome his desire for her, despite the terrible risk, every time - thousands of times before.