Death Eaters, being the new elites, are allowed to demand pretty much any wife (and it usually is a wife) they want. They can’t actually break up a marriage, but they can usually have the husband discreetly killed, so even being married already isn’t protection. The rest of the purebloods are left to their own devices, although if they don’t get married by about twenty-one, the Death Eaters start trying to play matchmaker. They’re never really forced into it, but they’re under a lot of pressure.