I'm not usually the earwigging type and, if I did overhear this sort of thing, I wouldn't normally repeat it. That said, the couple in question seemed very comfortable with vigorously discussing it at a high volume, eventually from opposite ends of the train carriage this evening so I feel it's reasonable to ask the question here.
At what stage in a relationship should a partner disclose that they've had a vasectomy (or a tubal ligation, in the spirit of balance)? In this instance, they'd been together for 6 months and had been physically involved several times (the exact number was disclosed, over my head, at full shout). Both parties professed that they had no interest in children but she 'wanted to have the option' and they had used protection so I'm unclear as to why this was specifically cause for a lively debate (particularly across a carriage full of commuters).