When women hit on me, I hold my hand in a way where my ring is very, very prominent.
If they don't "get the message" I start casually discussing my children.
That being said: I love the tradition of engagement and wedding rings. It's not just about preventing cheating. I bought my wife a moissanite engagement ring because we both wanted to follow the tradition, and we both understood that diamonds are a marketing gimmick to convince suckers to part with large sums of money.
I spent a sum of money that we were both comfortable spending, and we're both very happy with our purchase. My wedding band is made out of precious metal, and I love it. Other people I know wear cheap bands from Amazon and are happy with them.
If you want to be in a lifelong relationship with someone, it's important that you're both on the same page about these kinds of traditions. You don't have to follow them, but if your partner wants to, or values them, it might be worth it to play along to keep them happy. (As long as you're spending money that you're both comfortable spending.)
It's not about stopping someone from cheating, it's about a social display of a big part of your life. For the same reason people get married - it's just a social display. For women it can have the added bonus of stopping men from approaching them so much
Anyone trying to hook up will look for a ring as part of the non-verbal social clues.
Rings being expensive or not is a different matter.