People on this sub are very focused on casual sex, but what qualifies a relationship as something casual or not casual?

For instance, in my current relationship, we enjoy each others company, the sex, and doing things together. But neither of us are planning for this relationship to be "the one" and we're just having fun while it lasts. I realized this is basically the same thing as an exclusive friends with benefits.

So how does something cross the line into "something serious"? Do you have to be planning for marriage or talking about marriage? Or is every relationship serious as long as it's called a "relationship"?

I'm curious about how all this is defined.