If you marry a foreign national you will be required to report that to your local security manager because all significant contacts/relationships with foreigners have to be reported. Whether or not you "lose" your security clearance will depend on the result of the background investigation that your agency, in this case DoD, will perform. I currently maintain a TS/SCI clearance and I know how valuable they are. If you have any additional questions, feel free to give me a call. The initial consultations is always free. Good luck.
First of all, thank you for serving this Country. It means a lot to me.
You can marry whoever you want- but as a member of the service, it is my understanding that you are the property of the US Government and they can do whatever they want. that means they can take away your clearance if they feel they need to.
You should talk to your commander as this is not a family law question, but a military one.
Best of luck and thanks for your service to this Country.