I do agree that the military offers far more benefits to people who are legally spouses than people who are “just” partners or girlfriends/boyfriends; there are also lots of companies that give preference to military spouses because they understand the difficulty of obtaining a job. The benefits are also pretty great. (Husband is prior Air Force.) However, I do say that that is not a good reason to get married before you are ready. I was in quite a similar position - my husband joined the Air Force the same year that I entered graduate school and was making some critical career decisions, I was 22, and we’d been dating for around 7 years. We didn’t get married until after he separated from the military some 4 years later. It wasn’t a deliberate decision; that was just the right time for us.
Anyway, there’s not necessarily a linear path between major and career - you can major in political science or history and do any number of things, including jobs that are flexible and that allow you to move around should you choose to stay with your boyfriend. So if you want to major in political science, or switch to history, with the goal of becoming a lawyer (but the realization that there might be other things you want to do as well) - then do that! You can explore career options by doing internships and working part-time, or volunteering, in different fields. Many future lawyers get certified as paralegals or legal secretaries and do that work for several years before going to law school. (That’s also a legitimate career in and of itself.) And there are lots of law-related careers that aren’t law that can benefit from a major in political science - like policy research, international relations, consular affairs, working with nonprofits or NGOs that interface with policy or law groups (like the ACLU, a union, an environmental firm, Teach for America, etc.) So if you love it, keep at it, and find a way to fit your job to your interests rather than the other way around.
It’s not that I think you shouldn’t base decisions on your relationship (I think it’s only natural), but I do think that you need to think about how much you are willing to give up and think practically about whether those sacrifices make sense. One of the best and most giving things my own husband did for me, when I got accepted to graduate school at the same time he joined the AF, was to tell me to select a graduate school not based on trying to stay close to him but based upon what was best for my career, because that would give us the most options later. We’ve done that for each other throughout our early careers and training - made decisions that sometimes result in some short-term pain/sacrifice but with the long view in mind.
You’re only 20. Some separation would be kind of terrible right now but let’s say - you go to law school and work hard and become a lawyer whose work is so in-demand that you can work remotely and then easily move around every 3-4 years for your husband’s military career. And then he retires when you’re 40, after 20 years of service, and you’re just hitting your stride in your career and can join a great job/firm/whatever because of that experience. Then the sacrifice of the first 5-7 years of grueling labor and separation are worth it, right? Also envision other scenarios - scenarios in which your boyfriend leaves the military after one or two tours; scenarios in which you break up with him; scenarios in which he doesn’t move that often (some military members do stay on one base for longer than 3-4 years - it’s not common, but it’s not rare either.) When I entertained these scenarios, every decision point pointed to me spending the time and effort to develop my career early on so that I’d be more prepared to handle the challenges later as they came up.
If you want to be a JAG officer, the military does try to keep spouses together. TRY is the operative word. Also “together” means “bases within 100 miles of each other.” The hope is that they are the same or adjacent bases. This is more difficult in specialized fields like JAG, particularly if your spouse is also in a specialized field. The thing to remember always always always about the military is that the needs of the [insert branch here] come first. If you stay involved with a military member you will hear that terminology A LOT, especially as they are changing something on you that you expected or that would make your life easier. Deployment date got moved up 6 weeks? Needs of the Navy come first. Husband gets deployed again just a month after returning? Needs of the Navy. Husband got promised a base in California nearby where you finally found a new job, but suddenly they need him to go to Nebraska (or Germany!)? Needs of the Navy!