Most everyday women that have more than a vagina to offer with a good head on their shoulders, somewhat educated, and decent or better job are not going to be content in that kinda situation forever. They are eventually going to want more.
Marriage and custodial custody of children do heavily favor women if things go sour and situations have to be mediated in court.
That being said when life starts getting hectic and you have to deal with real shyt like the death of loved ones, illness, getting older and the challenges that come with that, drama on your job, and other things you want someone in your corner.
The law doesn't even recognize longterm girlfriends/boyfriends in remotely the same manner that they do husbands/wives. You should familiarize yourself with successions (you might already be familiar with them). The law doesn't care about Keisha who you been sleeping with for 25 years. Your girlfriend has no rights to heir property, retirement benefits, etc.