Hi @erice,
Well, how exciting!
What an adventurous time for you and your girlfriend. Marriage and children are a big step but talking about it first is important, communicating/talking about this stuff is always important. Its heavy stuff.
I developed T1 after my husband and I were married so it came with the whole marriage “surprise” package- the things you develop or find out about your spouse after marriage, intentional or not. 
My husband and I had two kids before I was diagnosed too and I was 26 when I had my first kid. Remember the odds are not a sure fire 100% numbers to live by. Below is an section from diabetes.org on passing T1 to kids. I have read similar numbers on other medical websites regarding passing on diabetes.
“In general, if you are a man with type 1 diabetes, the odds of your child developing diabetes are 1 in 17.
If you are a woman with type 1 diabetes and your child was born
before you were 25, your child’s risk is 1 in 25; if your child was born
after you turned 25, your child’s risk is 1 in 100.
Your child’s risk is doubled if you developed diabetes before age 11.
If both you and your partner have type 1 diabetes, the risk is between 1
in 10 and 1 in 4.” -
I have to say that for men, yes, the risk is higher of passing on T1 to kids genetically- 1 in 17. I have a couple of male friends that have T1 that chose with their spouse to have only 2 kids because of this. My husband and I chose together to have 4 kids, diabetes or not. Although we did revisit this discussion after I was diagnosed. What is more important than genetics is that your girlfriend/wife are in agreement and decide together. You two decide what you can handle or potentially handle. My odds of passing D down is 1 in 100, we have 3 kids + a bun in the oven. My odds are low but even if all my kids got D then I know we could handle it. We have a great marriage partnership together. 
At some point, all parents with T1, I’m sure worry if their kid will get T1. I cannot tell you how many times I’ve had my kids’ pediatrician do, what to them seemed like, unnecessary tests but to me eased my worry when the results were negative. I’ve gotten used to letting go of the worry but still cautiously look out for signs of T1. Part of being a parent is caring and being cautious but knowing that things are going to happen good and bad to your kids and you may/may not have any control over them happening. Its hard to see happen, but it happens. Kids grow up and you cannot have a death grip on them.
I wish you and your girlfriend the best! I wish you the best with all this decision making too.
Busybee