"Move out. If he wants you back, and his DC, he can bloody well get over himself and commit to all of you legally."
The trouble with this is, it simply reconfirms to him that he's right not to marry her. If she resorts to threats like that before she's married, what's she going to do afterwards when she's got the whip hand and the threats have much more weight behind them? Does a leopard change its spots?
Unfortunately, this is zero sum - the financial security that accrues to the ex-wife comes wholly out of his own.
TBH, I find it quite odd that some otherwise intelligent women are surprised that men are increasingly reluctant to marry. If you make the consequences of divorce worse for men, then obviously this will make them less inclined to risk it, just like if you make it hard for employers to sack people, they stop hiring them.
It's all very well to say that it's cynical, mercenary etc to worry about a divorce before getting married. This is dead easy to say when you're the one who's going to get, say, 70% of everything.