@storm_crowThe translation is correct.
In wuxia, xuanhuan, and xianxia stories like this one a "furnace" is a person(usually a woman) a cultivator dual cultivates(ie has sex) with in order to improve their cultivation. So in these type of stories women(sometimes men) who are virgins are used as "furnaces" or sometimes referred to as "cauldrons" in order for cultivators to improve their strength by absorbing their vital yin(women) or vital yang(men). Its obvious the "marriage" thing is just a ruse to take Ning'er as a furnace so that guy can boost his strength. And yes there are gay male cultivators who “steal” vital yang from men.
Think of it like what a succubus(female sex demon) or incubus(male sex demon) in western fantasy does to its victims.
In most stories dual cultivation is usually seen as evil or immoral since most people practice the orthodox/conservative form of cultivation which is basically cultivating ones self by yourself.
However dual cultivation isn’t always evil, sometimes certain techniques require you to have a partner and you can grow stronger together than if you were by yourselves. There’s a novel called “Dual Cultivation” that has a story based on the non evil version. There is even another popular opinion one called “Grasping Evil”.
But in most stories dual cultivators whether man or women would be seen in a poor light due to the nature of the practice.