If you want a romantic argument for getting married, try this.
When you get married, as far as the law is concerned, what's yours is his and what's his is yours. You are officially a family.
You've made a decision (together, as a family, even though the law doesn't consider you a family) that you will work part-time to support your shared children and his career. This means that by not being married, you are trusting and hoping that everything will work out, and he has a sort of insurance policy against it not working out.
For the man - at least, the higher earning man who doesn't work time and doesn't compromise his career or pension entitlement - getting married is not a financially or legally sensible thing to do. So the reason to do it is because you love the person and want to be considered a family in the eyes of the law. For better, for worse, for richer, for poorer, in sickness and in health.
Quote the marriage vows at him. That's what it means and that's why it's so important.
What could be more romantic than him agreeing to get married, rather than keeping the door open to ensure an easy exit?