Thanks for sharing your question and experiences. What I'm reading is that you are focusing a lot on this woman's wants and desires but I'm not hearing anything about your goals for your life. Many red flags were raised by very experienced posters.
I was engaged in my early 20s to a girl who probably was less pure than your girlfriend seems to be, but we had sex and were exclusive. She wanted to get married but turned out she really didn't want kids. I did (at the time) and that turned out to be a deal breaker, mostly for me. More importantly, I realized a couple years later that I was investing 50-70% of my mental and emotional energy in this relationship, plus time and money. I was graduating university and starting a job but really didn't know what I was passionate about creating in the world. I was raised Catholic but couldn't accept party line on God, Jesus, etc. I had to find out on my own. I couldn't do any of this with a woman so tightly bound to me - or rather, me so tightly bound to 'needing' her.
I doesn't sound to me like you have strong masculine role models, and that's what this forum is good for at least in the virtual sense. But you need to find your mission, your passion, your purpose, and do that without another person's demands influencing you. Not to say she is evil, but women are biologically driven to manipulate men because they lack the drive and ambition to create their own destiny. You need to really get to a place where you know who you are as a man first and foremost, then decide how much to let a woman into your life. This all sounds like your goal to find a virgin which is noble but a third or fourth priority compared to really developing into a Real Man, the best man YOU can really be. You won't ever find that out with some woman putting time and commitment demands on you. You need to read Esther Vilar's "The Manipulated Man", free PDF available online.
Your thread title says it all - LTR wants commitment for marriage.
What, my brother, do YOU want from YOUR life? Why were you born into this amazing crazy inspiring possibility-rich planet? Your human life is a tremendous gift and as a man you are someone who can create incredible things from his mind, heart, and personal connection to God / Spirit / Highest Self. NO ONE else can tell you how to live as a man; that is for you to decide but you must guard yourself so carefully against those who would control, manipulate, and coerce you into fulfilling THEIR needs over your own.
No woman will ever give a fuck about YOUR life as a man even 10% as much as you do. Your mother might care a little more than that but she will still deep down be thinking / feeling "I want my son to be a credit to me, make me look and feel good and proud" and not focused on your development and fulfillment as a man.
I think many of us have been where you are. Please rub one out (or four or five) if you need to so you don't have a lust / hormone-induced fog clouding your mind and take a weekend away from contact with her and EVERY woman to discover yourself. Not just one weekend but many. Take a drive, go for a solo hike, be in nature and bring a note pad and pen, or even a folder over piece of paper to write notes on. Turn your cell phone off! Just be with yourself. There is a man inside that is blossoming and growing and your must nurture that seedling, that tender sprout that wants to grow into a great redwood tree. We are the grove that can help nurture you and your safely and understanding. I didn't have this when I was in my 20s (early 40s now) but I had a sense of mission and purpose that was compelling me, plus could see unhappy people around (most of whom were married) so I knew I had to get to know myself before committing my life to a woman.
I would say let this woman go and focus on yourself. Find your campaign as Roosh blogged about, find your purpose - I link to a great book "Is Your Genius At Work?" that helped me discover mine, and spend more time as a solitary man. You need it. I also recommend Rainer Maria Rilke's "Letters to a Young Poet" which contains so much simple wisdom as he endeavors to live truly and honestly. The letter #4 also addresses marriage, though all are very worth reading and contemplating:
Main list:
http://carrothers.com/rilke_main.htm
Letter #4:
http://carrothers.com/rilke4.htm
It's not about age only, but you are - from what I'm sensing - more prioritized on her qualities than yours as a man. Trust me, you will find better women who will respect you for having deeper clarity and purpose than you do now. I met a woman who was 20 when I was 35 and she was the most beautiful, tender, and devoted girl I've ever known. I treasured her, loved her feminine energy and showed her tenderness and also my passion for life and her. She was not a unicorn per se (we are not together right now) but she respected my knowing of myself and wanted to join the adventure as long as she could. The time we shared was a great motivation to me to continue improving and growing and I feel now years later I am a better man all around.
By waiting and deepening in your knowing of who you are and becoming your best self, you will have a more wise perspective about women, dating, marriage, life - if you wait. Your perspective even a year or two about all this happening now will be wiser!
Keep in mind that anyone - ANYONE! - PRESSURING you for commitment wants to saddle to your horse and take advantage of you. Commitment being DEMANDED is the #1 and most critical red flag. Guard against those who would turn you into their plough horse, their mule!
As others have said with fewer words, don't do it!
Watch videos by MGTOW is freedom, just for perspective. John knows about marriage and knows men need to know themselves. You need to live much more life and learn who you are before you even consider a legal and emotional commitment so serious as marriage, and only when you are fully aware of the dangers and ready to deal with them.