I have been with the same girl since 1977. I married her in 1980. We raised three kids together (actually, SHE raised three kids - I checked in once in a while).
I'm retired now, but served from '77 - '80, and then again from '82 - '04. During that time I did eight tours - where doesn't matter, but not all of them included LTA, or phones. At one point I was away for a year without a an LTA, VTA, or other "reunioun" benefit, and there were NO telephones provided for me (I had access to civilian facilities, at civilian prices - OUCH!!)
My girl stuck with me - and I remained stuck on her. I had "opportunities" - especially when I was Airborne (that old Maroon Beret seemed to be better panty solvent than booze OR chocolate)!! That I did not "take advantage" of those opportunities was because I was (and am) totally dedicated to the girl I married. There was a time when (false) rumours reached the home front, and the poor girl (raising three toddlers at the time) had to accept my word against "theirs" - it wasn't easy then, and I don't imagine it's easy now.
Bottom line - most men and women I have served with are people of PRINCIPLE, in ALL aspects of their life - they lay their lives down for principle. What makes you think that they'd forget those principles the first time some girl/guy "shakes their booties" at them??
Are there duplicitous idiots in the military? Absolutely. Are they the majority? Absolutely not.
Don't worry about your man/woman - he/she remains themselves, deployed or not. If you've got to worry about him/her when they're home, then you've got a problem. Distance will not change people - it simply reinforces their strengths and weaknesses.
Don't know if that helped or not - but that's my two cents.