Eeehhhh.. From the outside it looks like the relationship has plateaued.

You're both feeling unfulfilled in the relationship, and it's come to the point where she's issued an ultimatum, which is very unfortunate. Just play out the scenarios long term. If you just need a kick in the ass that's one thing, but you've gotten the kick and it doesn't look like anything happened except it upset you.

First big question to discuss with yourself: Do you or do you not want to get married? And do you or do you not want to marry her, specifically?

Second question: What will marriage change about your relationship? Do you believe that agreeing to marry her will unblock and resolve all your tensions and let you move forward?

Usually that's not the case. There's a bad stereotype about bad relationships getting "fixed" by getting married. Then when the toxic relationship becomes a toxic marriage, they "fix" that by having kids. And now you've got a couple stuck unhappy with a family that didn't give them the happiness they want.

FWIW, I'm the same way about kids. Been married almost 3 years (together for 12) and I've always wanted kids "some day". Not today, not tomorrow, but I wanted that door open, and it bothered me that my wife was a lot less open to the idea of starting a family than I was. But I let it fester, and sometimes it really bothered me. I wanted that door left open, because deciding to not having children is so final. And yet, after a decade, "some day" still hasn't come. And it's not going to. But it took me a long time to be really honest about it.

Just make sure whatever you decide is what you want, and you the consequences of that decision for the rest of your life. Keeping things happy is important in life, and the importance is in order of how close they sleep to you. Keep your brain happy. Then keep your wife happy. Then worry about everyone else, because their ability to make you miserable is far less than the other 2.