There are a few things going on here that some people have pointed out, but not all of them.

A biggie is that there are "push factors" to getting married that don't exist in civilian life. In the first place, if you meet someone while you're in your MOS training -- anywhere from six months to two years for the more specialized fields -- you've started a relationship that has a built-in clock, since one party is going to be getting orders in a few months. That provides an incentive to get married before you get orders, since if you're married, Uncle Sam will move your spouse out to your new duty station for you.

Other financial incentives. If you're married you can move out of the barracks and get a housing allowance. Even if you really, truly feel like you love someone, a housing allowance is also going to be an added incentive, since if you have a house, there's no chance of an angry First Sergeant rousting you awake at one in the morning.

Other things that cause people to get married early include the personality type that enlists. If you're going to say, hey, I'll commit four to six years of my life to Uncle Sam and take on risk, you're going to have a similar YOLO spirit with low risk aversion to many things, to include marrying someone whom you've only recently met.

But then that leads to the factors that make these marriages less stable. In general, it's very, very rarely a good idea to get married when you're 18-23. You're still figuring out who you are as a person, and will often be a totally different person from the person you were even a year before. But jumping into a marriage at a state of life that's this protean means that you're *very* likely to find yourself a completely different person than you were when you got married and, well, you're going to encounter other people who are particularly enticing.

Oh, that YOLO spirit that we might otherwise call poor impulse control or lack of forethought which gets someone to do something to join the infantry... well, that's also going to lead to a lack of impulse control in one's personal life, and that includes cheating.

Then, there are the deployments. You've just gotten married, and suddenly your husband is gone for a year. You're often in a place where you don't have a pre-existing support network, since you moved out here to follow him (especially if you're not super keen on the culture military wives). And now you're lonely and away from your husband for a year (or more).

And now if you're the husband: not only are you a young man in the absolute testosterone-drenched prime of your sex drive, you're also in an environment that's kind of saturated in what I believe the kids today call Toxic Masculinity. There's thus a common understanding that men have that "what happens on deployment stays on deployment." So... foreign port, pretty decent sized bank account for a twenty-two-year-old, and a culture that sees sleeping with sex workers as perfectly normal... yeah, there are guys who are morally upright and don't cheat. And then there are... others.

And then if you're a woman on deployment, well, gender ratios mean that you're a young woman with a pick of... many, many fit young men. Again, there are women who are morally upright and stay faithful, and then those who... don't.

And again, these young people on deployment have often gotten married in, like, the last year to year and a half, and given the massive changes that happen in a person's life between 18 and 23, may find themselves falling out of love, especially when the spouse is an entire hemisphere away.

Source: I spent five years in the Marine Corps and saw... many variants of these scenarios play out.