So when is he scheduled to come back? Soon I hope?
Brian sounds like a genuine stand-up guy from what I can tell. He is in Iraq defending his country and may not return alive. He's doing the honorable thing by letting go of the woman he loves so she can find someone else to take care of her while he can't or may never be able to. Brian seems like the true definition of generous.
Brian also loves you immensely from what i can tell. If he was selfish, he would keep you waiting for him while you never know when he'll return, if even. Moreover, you're an adult and it's understandable that you need/want sex. It's human basics and he seems to be the type of guy who will be happy for you as long as you had a good time (I know it's hard to believe and I can't explain it but he just seems to be one of those guys.)
You said you will regret 5-10 years into marriage if Brian is the only man you've ever been intimate with, then think about being in regret 5-10 years into marriage with a secret haunting your marriage. What if that other marine somehow finds a way back into your life 5-10 years into your marriage with Brian? Think of the other side of the coin.
I say there's only one question that needs to be answered, "does Brian still love you a lot?" If so, be honest with him and even tell him everything you've told us here. You'll be surprised because he may totally understand your situation and won't feel hurt about you "hooking up" with another guy.
One advice though, maybe wait until Brian returns from Iraq to come clean so to avoid throwing Brian off focus on duty.
















