Pregnancy makes you very vulnerable (16 weeks myself at this time) and you want a form of security knowing that childbirth and aftercare will be tough. It's very straining on the partner as well, you want someone fully committed. Both emotionally and financially.

I could never force this on an early stage on someone who is not the father. That's harsh. Also, thinking about the kid....I want someone around long term and not someone with doubts or "obligations".

It's selfish to trap her in a scenario of marriage which you already say you are doubting. You see yourself as a white knight hoping to sweep her off her feet once more cause you think you can't get any better. But you can.