In a relationship how does a couple know to have sex?

the husband & wife wait til their wedding night............not that difficult to figure out

it's not to hard to know how. As far as when, whenever they both feel ready. i would withhold for awhile personally.

it just....happens

The couple is not a person. It's a mental construct that doesn't have any self-awareness or knowledge.

Only individual people can know something. Which in practice means that one partner initiates foreplay and sex, and the other partner either goes along, or stops it, depending on the feelings and the circumstances.

they don't actually very few couples have sex