I don't have an opinion on whether you should get married, but I do know there are significant potential financial benefits to being married as opposed to simply living with a person. These may or may not apply to your situation. Some of them include:
Spouses are eligible for social security spousal benefits while alive and survivor benefits upon the death of one spouse. These are most relevant if one spouse has a significantly higher SS benefit than the other.
A 401k or IRA has better RMD options when left to a spouse as opposed to a non-spouse.
If you live in a community property state, if you elect to make some of your assets community property, a surviving spouse gets a full step up for the community property. So, if John and Mary each have $100,000 in non retirement assets each of which has a $50,000 basis and they combine their accounts and declare them community property, then upon the death of either, the survivor has $200,000 with a basis of $200,000. Better than $200,000 with a basis of $150,000 which would be the case if they did not get married and left the account to the survivor.
In California, a spouse may inherit a residence's property tax basis from a spouse. This could be a very valuable benefit due to Proposition 13, if one spouse has held a property for a long time.
Many other potential benefits related to employment and insurance.
Sorry but I would like to go on a little rant: I often hear or read derogatory comments directed toward couples that opt for a prenup. When I married my spouse, we had a prenup executed. We got married fairly late in life and will not have children. We each have our own assets that we would like to see go to our own relatives when we are both deceased. With the advice and help of a few attorneys, we put a plan in place to do this by executing a prenup wherein we keep separate property (including our retirement accounts). Our estate plans are coordinated with our prenup such that a surviving spouse can use the assets of the first deceased spouse. Upon the death of the surviving spouse, the unused assets of each spouse pass to each spouse's designated beneficiaries. My point in saying all this is that for us a prenup was crucial to assuring our wishes have the best chance of being executed. Anyone who tells me prenups are useless or taint a marriage is in my opinion displaying ignorance and intolerance. There are many other reasons couples elect to have prenups and my opinion is that as long as a couple is in agreement on the terms without coercion, no one is qualified to pass judgment on their choices.
Regarding the enforceability of a prenup, we were told by several family and estate attorneys that in California it is very difficult to successfully contest a well executed prenup. In California, the standard for throwing out a prenup is that it be "unconscionable", or not have met some statutory requirement such as each side having their own counsel et. al. Now, if one of the parties is a billionaire, or one of the parties is being left with meager resources, the calculus will be different. But if a couple has a prenup executed properly with reasonable terms, it is very unlikely to be contested at all, let alone contested successfully.
Anyway, to the OP, be aware there are some financial benefits only marriage can confer. A properly executed prenup is a legal option to substitute mutually agreed upon divorce terms over the default in your state and no one should feel their marriage is any less if this is elected. Finally, best wishes to both of you whatever you decide!
All this at age 50. (and my ex also got 1/2 my meager ARCO pension that was vested) 
