I don't agree that just because we've stopped explicitly mentioning religious or sexist things (or religion) in the ceremony of marriage that it exempts the whole institution and custom from being patriarchal in nature. The whole notion of marriage as a custom is rooted in concerns about property ownership, control of women and legitimacy of children which is all about protecting men and men's rights, and even the so-lovely 'protections' of marriage for women that are vaunted around a lot, are grounded in the assumption that women will be negatively impacted economically by childbirth and childrearing, will be financially dependent on their partner, won't have assets of their own etc. You only have to look at the most common traditions and customs that prevail around marriage (and yes yes I know you don't have to but the majority of people do the majority of these) to see it's definitely still sexist, the woman wearing white, the bride being given away or accompanied by her father, the men of the party (father of the bride, best man) doing the talking while the women stay silent, the woman changing her name while the man keeps his, it goes on.
It doesn't have to cost money to be just as 'protected', obviously it all depends on circumstance but for me the biggest way anyone can protect themselves is for both parties to retain an income as equal as possible, keep assets and savings in joint names or equal shares (or unequal shares if one party has given up remunerated work to raise children, a greater share of the capital can be put into their name as protection, depending on what the asset is this isn't necessarily expensive to do, e.g. houses it's simple tenants in common with a deed of trust explaining the split). Then if there is a break up it's if anything much easier and cheaper to manage compared to a contentious divorce as both parties simply keep their own assets and incomes (may still have to sort out maintenance of the children but that is an issue if married too)...
Wills can be made to avoid intestacy issues which you should do anyway even if married. Powers of attorney can be a good idea to protect against loss of capacity in old age (and again still a good idea even if married). Pension schemes and insurances will usually allow registration of an unmarried partner as beneficiary with no cost. The bit you can't really work around is the tax issue, which again isn't about it costing more to make arrangements, more that you pretty much have to get married for the state to recognize your relationship...
I'm not saying all this to put down or criticise those who do choose to get married, it can be a perfectly sensible and reasonable step, or to deny that there's useless commitment phobes out there who use excuses about not liking the institution of marriage to shirk their responsibilities. But I do think simply saying oh silly billies, modern marriage is nothing to do with the bad old days, just get married, is minimising the fact that some people genuinely do feel it's not something they want or need in their lives...