I've been with my partner for 8 years now. We've owned a home together for 5. She and her ex have yet to get divorced. There's been no meaningful impetus to drive them to do it: It doesn't change their relationship any further, it doesn't change her relationship with me at all, yet it costs a couple thousand bucks.
Why spend that on lawyers when you could spend it on car repairs, home upgrades, a plethora of takeout meals, or a vacation?
Both ways. He should live it up for a couple of years. Make sure to use her line of credit and credit cards. His wife’s ex-husband will have to pay half when they finally divorce. Buy a car in her name, sell it, and use the proceeds to buy a car in your name. The possibilities are endless.
[I am not advocating this obviously. Mostly I am agreeing how super dangerous this is.]
Marriage has all kinds of default legal implications ranging from property ownership to inheritance and tax benefits, and may also affect insurance rates.
How you are affected depends on where you live and your personal situation.
Frankly, a blanket statement like this is completely irresponsible. Consult a lawyer on these matters, people. Or spend tens of thousands of dollars later in life dealing with the consequences of ignoring your legal rights and responsibilities.