El is past his 30s. He can elaborate if he wants, but I think it is safe to say that he's "established," if you know what I mean? He's not just starting out in his adult life, he'd been through buying a house, marriage, jobs, all of that in America before coming here.
I think it is one thing to come to China in your 40s, 50s or 60s and decide to stay here forever, but it is another thing entirely to do it at 24. And I say this as someone who came to China at 23 and has been here for 10 years now. I have spent pretty much my entire adult life in China, and I will just say that if there is something you want to accomplish back home, be it a PhD, a career, whatever, it will be much easier to accomplish it now than it will be 10 years down the road when you have a possible family to support. Even if the plan now is to stay in China, you may find yourself in a position at some point down the road when you need or want to return to America and you don't want to be stuck with no qualifications and no marketable experience.
I will say that I used to make the decision to stay in China every year. It was renewable. But when I got married, that wasn't the case anymore. China is going to be home until one day we decide it isn't. And it might never come to that. Anyone who is getting into an international relationship/marriage needs to realize that there is always going to be one party who has to be away from home, possibly forever. I will say quite honestly that I don't think it is fair for either party to take either country completely off the table. This is going to sound harsh, but I think if you're going to be inflexible about where to live, you probably shouldn't marry someone from another country (I suppose unless that person also has no desire to live in their home country). It isn't fair to say "I want you to make this ultimate sacrifice for me but I am not willing to do the same for you."
Luckily, I think Casey is right. A lot of people say no initially, but when you have a family you put the family unit first and I think most are able to compromise their individual wants if a move to the other country is in the best interests of the family. But this is something you should really sit down and talk with her about. If you're going to be a unit, a real unit, there needs to be compromise on both sides.