It sounds like a lot of your thoughts on marriage and kids boils down to your "plan for the worst" mindset. You're not fond of marriage because of the permeance of vows and the volatility of the future. You're not fond of having your own kids because of the genetic risks, the physical impact on your GF, and the life changes that come with a kid.

To that I just want to bring up a point to consider: A lot of life has its risks, but to a lot of people the pros outweigh the cons. Talk about it with your GF but be prepared for her to be not okay with it. Beyond that, I think it's worth reassessing your risk-adverse views on relationships.

To put it another way: It doesn't sound like you're an agoraphobic shut-in. Yet leaving the house puts you at risk of automobile accidents, a leading cause of death in the US. You presumably also don't have black-out blinds throughout your house and leave in a thawb despite cancer being an even higher cause of death and skin cancer heavily influenced by UV rays.

IMO it's worth trying to reframe your views on relationships in this manner. You're right - some marriages do fail and some pregnancies do end with physical harm. That said, by focusing on the negatives, you're throwing away the positives that have such a significant emotional impact on your GF. For her, the risks don't outweigh the rewards.