Sorry you are having such a stressful time when you both need to be concentrating on your pregnancy and getting ready for the baby.
I know being married is an emotional thing as much as a legal and financial one, so this will possibly sound very cold but... Knowledge is Power and you might feel more in control of your situation if you thoroughly researched the financial and legal side of marriage vs cohabitation. And your rights if you separate. Or if one of you dies.
There are some good websites which are a useful place to start eg
www.marriedornot.org.uk/
Your local citizens advice bureau might be able to advise you if you'd rather talk it over with a person.
I'm out of sync with most of the replies on this thread because - at least for the time BEFORE your pregnancy - I do actually see where your DP is coming from a little bit. My DP and I also started our relationship financially unequal (we've been together about the same time as you) but we're the other way round to you, as I'm the one with the higher salary and assets. My DP is a wonderful and amazing human being and is mad enough to quite like me too, and we are committed to each other and very happy... and we hope and expect to remain together... and we're talking about children... BUT rationally, we both know that some couples do separate and marriages end. My parents' marriage did. We think it's fair that if that happened, and while there are no children in the equation, we both take away what we brought/put in (financially and otherwise). If we were married (which we aren't) this would be in the hands of the courts, which have a wide discretion on how assets are divided. So we've done as best we can to achieve what WE want and believe to be fair, by drawing up a deed specifying our respective shares of the house (our biggest asset), sorting out life insurance, and making wills so neither of us is left high and dry if the other dies.
If/when there is a baby to provide for we will need to totally review our finances anyway. Providing for the baby would be the number 1 priority. This may mean marriage. It's something we will talk through as and when the time comes.
Maybe it would help your situation if you sit down with your DP, specifically NOT to talk about marriage as such, but - with business head on - to talk about putting your financial affairs in order as a couple, now your (HIS!) child is on the way. For example, what if he were to be run over by a bus? What if you were?
He might respond better to this approach and anyway, as a couple, it's probably something you'd need to do anyway if you haven't already. Putting things in order might turn out to be by marrying... or there might be other things you can both do, which at least protect you and the little one a bit better.
Good luck, whatever you decide to do, and I hope it all works out the best for all three of you.