Live with longterm gf; I earn roughly double what she does, we both have our salary paid into separate accounts and a joint account for bills (which I pay more into). I have more disposable income to spend, but will buy her things or give her money if she were to need it (which she never does as she gets a decent wage).
If we were married it would likely be different, if we had kids it would certainly be different, as I wouldn’t want it to feel like she had to come to me for hand outs if she was looking after the kid.