First off, congratulations on the proposal! Your situation right now actually amazes me since I rarely hear of familial marriages that continued from grandparents. I believe the first thing that you would like to ask yourself for your marriage is how much you love your to-be husband, how comfortable you will feel about it, and whether your other family members are also comfortable with it. It's important that the two of you have people that are understanding and willing to support your marriage even if you're cousins, because other people in our society tends to be very critical about it. If there's no such problems on those fields, then I don't see why you two shouldn't be together.
Your concern for the child is another issue, as there are a number of options available for a couple to have a child. From reading your post, I'm making an assumption that you and your husband-to-be wants a child of your own but you're afraid the child might turn out having defects as well. I'd have to agree with Synalon that reiterated instances have a greater chance of negative consequences but ultimately, you'll never know the outcome unless you get pregnant since you yourself are born normal despite having parents and grandparents that are related. There are ways like prenatal genetic testing and chorionic villus sampling that enables you to know whether the child you conceived have abnormal genetic traits. There are issues surrounding this though, you can read more about it here: Debate continues concerning ethics of prenatal genetic testing. Personally, my stand is that this process is not to make the option of abortion available in case there are detected abnormality, but simply to give you and your husband a foresight in your upcoming parenthood so you'll both have a time to prepare, understand, emotionally be ready to accept the child in whatever condition he/she's in, and be the best parents in the world for them.
Some couple choose adoption when they know they don't have the capability of having a healthy child due to physical or gender issue, or they run high risk of conceiving a child with defect or abnormality. Others approach the institutions that could help them find sperm/egg donors (as is the case with some lesbian couples and partner suffering from infertility). Or, some even choose the road of not having children at all. The point is, instead of giving up the marriage right off the bat, it will be better if both of you will have some intimate conversation about this. Sorry for such a long reply as I'm also a woman and can't help but feel concerned about the issue of pregnancy, but I hope the best for you two!
