From what I've seen marriage ruins sex. For example, my friend Tonya has been married twice. She lived with the first guy 3 years, they divorced a bit over a year later. She's been with husband number two 9 years. They married this year and are having major problems.
Marriage blows.
Just because marriage doesn't work for some people doesn't mean that it's bad for every person on Earth. I hate this generalization. My parents have been happily married for almost 45 years. The idea that marriage ruins a relationship is just fatalistic. It's saying "I'm positive this relationship is going to fail, so I'm not going to commit to it in any iron clad way."
I'll grant that maybe for some people, marriage has this odd relationship ruining effect. It sounds as though that could be the case with your friend Tonya. I don't really understand it. Maybe the marriage act flips a psychological switch in their brain and tells them "this is it. this is forever", and they freak out and feel trapped. The point is that this is just her personally and not a valid indictment of marriage itself.
The thing is that it could cut the other way. I don't think I could stay with a woman who told me "I don't want to get married; I don't believe in marriage. Can't we just live together in a monogamous relationship for the rest of our lives?" I wouldn't be cool with that. I'd feel like the woman didn't sincerely want to stay together and just wanted to keep an easy escape hatch open because she didn't believe in the relationship. I just can't see an advantage to a long term monogamous relationship over marriage, except that the prior one is easier to get out of. And if you prefer a committed relationship because it's easy to get out of... it's not a committed relationship in my opinion.
There's also the ever present question "Oh we love each other, why do we need a document to prove it?" I always say "If you love each other, why is a document proving it such a big deal?" It's like.... I know I was born, so why do I need a birth certificate? Well getting a birth certificate is not difficult.