He would be the lover of a married woman. Since most modern men would simply divorce their wives in that situation, I don't think that there is a contemporary word for a man who makes another man a cuckold. But, According to Webster's Dictionary 1828 edition: CUCKOLD-MAKER, n. One who has criminal conversation with another mans wife; one who makes a cuckold. From the 1913 edition: Horn maker :a maker of cuckolds. (that is because horns are the emblem of the cuckold)

Cuckold is a man whose wife has been unfaithful to him " a man who makes another man a cuckold" means a man who make love or having sex relationship with a married woman. Background: The allusion to the cuckoo on which the word cuckold is based may not be appreciated by those unfamiliar with the nesting habits of certain varieties of this bird. The female of some Old World cuckoos lays its eggs in the nests of other birds, leaving them to be cared for by the resident nesters. This parasitic tendency has given the female bird a figurative reputation for unfaithfulness as well. Hence in Old French we find the word cucuault, composed of cocu, "cuckoo, cuckold," and the pejorative suffix -ald and used to designate a husband whose wife has wandered afield like the female cuckoo. An earlier assumed form of the Old French word was borrowed into Middle English by way of Anglo-Norman. Middle English cokewold, the ancestor of Modern English cuckold, is first recorded in a work written around 1250.

A player. Kidding, but "player" is used colloquially for someone (usually a man) who has many romantic liaisons. However, it's definitely not specific to someone who sleeps with another's wife. "Adulterer", someone who commits adultery, would be better, but an "adulterer" may only be betraying his own wife, not necessarily making a cuckold out of someone. Hmm. There may not be a specific word. Does anyone know?