As another perspective, I wonder how old you are now? And do you think you're ready for marriage now?
If you're expecting to be head over heels in love so you'll feel it strongly enough to want to go ALL the way, then you can ask yourself if you felt that way at first, and how long ago was that? How long have you been together? Very important, because for many people, if you stay with a person for 2 to 3 years without escalating to marriage, the passionate excitement begins to simmer down, because we're biologically designed to produce a baby by then and then either "move on" (in a polygamous or serially monogamous society like rural Trinidad) or begin the householder years by raising the baby.
Furthermore, your wishful interest in the new woman fulfills the medieval love rule: "New love chases away old love" because of the mystery and uncertainty stimulating your excitement.
If you've been together around 1- year or a little more, then you've probably found out some ways you don't fit as perfectly as you once thought, and possibly some ways you irritate each other. And she doesn't seem like your ideal woman anymore--because no real woman can ever seem that ideal for too long.
And instead of just thinking about whether you love her enough, think about what you'll be doing 5, 10 and 20 years from now, and what you think she'll be doing--do those projected life directions match pretty well?
Were there any other ways she has disappointed your hopes for your mate besides not wanting children at first? Do you think she'll be a good enough mother now, or that you might have to be more engaged (in time) with your kid(s) than you want?
That's some more questions to consider. And if you do ask for a sabbatical-separation to find out if you miss her and that makes your love grow stronger--then don't test out the other woman unless you want to risk hurting both of them, and messing up your future relations with both also.
You're very clear-sighted to realize that maybe you just never want to hurt anybody, so you can't make a choice that would hurt. But not choosing IS already hurting both of you.
OOOh! Something else just hit me! You know that she wants you to marry her, so now you're guilty if you don't marry her, but you're also NOT choosing to marry her because YOU want to, but because you aren't allowed to HURT her, by your own rules, that is. So you can't be true to your own feelings if you choose to marry her, because your guilt is preventing you from the enthusiasm you might otherwise feel. Your unwillingness to hurt anybody (did you grow up in a family in which "we mustn't hurt (Mother)(Disappoint Father)(etc)"? Is a dark cloud that keeps you from feeling your feelings.
I gotta go to bed. Ponder that and write back, if you want to--otherwise go ahead and agree with Dr. Paige and don't worry about all my additions. I think your "Hippocratic oath" is messing up your feelings.
If you can figure out WHOSE feelings you MUST NOT hurt (by your own, or by family rules) , write that person's (or those persons') name(s) down. Then write a letter to each one in which you list everything you've ever done that could hurt or disappoint them (or reap their disapproval) if they knew about it. Then go on and add some other things Much Worse that you could imagine doing, if you really Didn't Care about their reaction--go ahead and BE MEAN.
(You're not going to do any of those brand new meaner things, and you're not going to send any letter to anybody. But if you really take the time to immerse yourself in reversing your imaginary rules that you can't hurt (X, Y or Z), you just might set yourself free--a little bit at least.)
Then some time after that (at least after sleeping alone afterwards for a night or two) see if you can feel a little bit more of your full pallette of feelings for your girlfriend. Let me know if you do that (but not anything you wrote).