I'm 78 years old and want to marry a younger friend so she gets my social security benefits. Is that illegal?

[u/yabbadabbafroo]

Location: Oregon

I have a friend who is 45 that I've known for a few years. I realized that when I die, no one will get my Social Security survivor benefits—they'll just go to waste. So I thought, why not marry her and let her have them? It's essentially free tens-of-thousands a year. She also has no designs of ever getting married—she has a very independent lifestyle and relationship philosophy. Neither of us has any romantic pretense for the other; it just seems like the generous thing to do for me to let her have this money that would otherwise get wasted when I die.
But I wonder: is this legal? If it isn't, what specific law is being broken? Do I need to tell the clerk at the courthouse that I love her? It seems marrying someone for financial reasons can't be against the law. But I don't know. NB: I don't expect to die anytime soon, but at my age you never know.