I know many married men, I even know a few happily married men, but I don't know one who wouldn't fall down the first open coal hole running after the first pretty girl who gave him a wink.

In Nathan’s characteristically sardonic, anti-sentimental style, the line mocks the ideal of marital fidelity by portraying male desire as reflexive and easily derailed. The “open coal hole” image suggests not only comic slapstick but also self-humiliation: the man’s pursuit of novelty makes him literally stumble into danger. By claiming this applies even to “happily married” men, the speaker implies that contentment and commitment do not extinguish appetite for flirtation, and that social respectability often masks a more impulsive reality. The exaggeration is central to the effect—less a sociological claim than a satirical jab at romantic pieties and masculine self-control.