It does not seem that the issue of blood being - or + should make a difference if you love your possible bride. As Marthaelisa says, the doctor will give your wife a shot after she has the baby and there will be no problem for the next baby. I am just concerned about why you seemed as though this was enough to end the engagement.
That's not something that is too big of a deal, All the woman has to do is get a shot . This is called the RH negative factor. This is why it's important to know your blood type. With my daughter I was negative so I just got a shot and everything was fine. If you have that concern just tell your doctor, The whole crucial part is that if the woman does not get the shot her blood will fight against the baby's blood and that's not good. But it is something that is very treatable. Hope that helps.