I have a brother in the military - so first off - thank you for your service ! Second off, my brother also had a hard time breaking it to the family. My mom took it especially hard. She came to me crying that "your little brother is going to war ! He has to kill people!" ( she is a little high strung) (Not trying to shock you there by that statement, and although broad - even though you will be saving lives - you will always be a solider first.)
I explained to my mom that not everyone believes what you believe and that if we did not have people that went into our military, she would not have the freedoms she has. I emphisized that my brother was a "good kid" and that she raised us well.
I was not there when my brother broke it to my family, but he invited his recruiter over to explain to them the benifits and realities of it and perhaps this may help.
I think carla (above) explained it very well. I wish you and your soon to be bride the very best. Your country thanks you.