Yanman, I had two children with my ex, we never married, and I didn't think it important at the time. Then he dumped me and left me to bring them up alone, an 'unmarried mother'. I did'nt much care about it from a religious point of view but as a legal and social situation, it couldn't have been less fun.
When I met my dh neither of us had planned on marrying. I knew he was the one very quickly, but he kept talking about having a child with me and being with me forever, but did not mention marriage. I was surprised at how much this upset me, and it didn't take long for me to work out that now I had met the right person, I did want marriage before I would even think about having another child. Once he realised this, we talked and sorted it all out. We had a fairly long engagement, so we didn't rush things, but we had a plan and we were engaged, so I felt safer and also valued for the first time in my life.
13 months after our marriage we had a baby, and we are still very happy now three years later.
I am just telling you all this because I am thinking your girlfriend might be afraid of not only becoming pregnant, but of being left an unmarried mother with two children by different fathers. This is not a situation one wants to be put in, and even if the possibility is only slight, I can understand her concern.
I was tired of being used as a breeding machine and dumped (not that I'm accusing you of that, of course). I wanted someone to get things in the right order, love me, win me, marry me, then have a baby with me, because I needed that value placed on my life by my dh before I was prepared to fulfil all his wishes.
If you love each other, perhaps she would like you to talk about the future, possibly marriage, and help her feel a little safer?