First thing you need to do is to visit base legal and get smart on all the federal requirements. There are very specific, significant and long-lasting obligations you will incur when marrying a foreign national for the purposes of naturalization. Did you know that if things don't work out, you will be required to support him financially for 10 years? Bottom line, you need to fully arm yourself with all the information. Marrying a foreign national will affect your clearance, but not in a big way - lots of people do it. The amount of hassle will also depend on his country of origin. Obviously a Cuban or Iranian national will be more problematic than someone from Canada or the UK. Be smart and don't attempt this without proper legal counsel.
Brett
I got married to a foreign national last year, a Canadian. I went about things a little differently than you, I put in for a fiancee visa while she was still living in Canada and we waited to get that approved before we got married. I had the luxury of time though.
Like Brett said, go to base legal and find out what the Navy wants you to do before you marry a foreigner. Another good place to go to is the base SSO, the guys who handle security clearances. They should be in the base phone book and they should know what you need to do in terms of security paperwork. My wife had to fill out an SF-86, the same form you fill out for a TS clearance, partly because I have one. It is veryimportant you do this as soon as you can, you have to get approval from the Navy before you can marry your fiancee.
The Navy part should not be all that difficult with a French national, unless there is something to raise a red flag like a criminal or foreign intelligence background.
A big pain is going through the visa approval process. It is easily the worst government red tape I have ever had to deal with, along with close to $1000for all the fees associated with processing the forms. My wife has known several people who have spent around $5000 for an immigration lawyer to assist them in the process, money well spent for most of them. I was able to wade trough the forms over 2 months and get everything right the first time around. Even though my wife learned the Queen's English and has a BA in Communications, she had a lot of difficulty getting through many of the forms. A lot of them were like a bad joke "If you are applying for a fiancee visa and have no dependents, go to Section 16, sub-section e, paragraph 3, part 7" and other things like that. It was a real pain and only because I am used to government forms did we get stuff right. I have no idea how non-english speaking people do it. We still have a lot to do in order to get her citizenship.
Oh yeah, I have never heard of the 10 year support rule and don't remember seeing it in the forms I did.