He might have gotten lucky with respect to timing. He apparently joined the Army after college in 1987. He would be in the zone for promotion to O-4 after 10 years of service, or in 1997.
“Don’t ask, don’t tell” was implemented in 1993. Clinton prohibited sexual orientation discrimination for security clearances in 1995. Even if his background check revealed he was gay, it could not be used to separate him from the military or deny clearance.
Assuming he did not require security clearance before reaching Major, his sexuality was not an impediment. He could have been separated had his CO had some independent source of homosexual activity since he would not have been privy to the background check.
Also, who knows if he was an active homosexual during that time? He might be bisexual or was asexual for a time.
Requirements are usually a Secret for 0-1 through O-3 unless MI, SF, Gray Fox, some DOD and Joint Command jobs and some other areas. TS for 0-4 and above, although some have made 0-4 without it. Req before CGSC.
Crypto is another issue.
Secret can be granted by National Agency Check alone, but TS requires a full SSBI. He might have made it through thanks to Slicks catering to Hilary’s pro-gay demands
BTW, Above Secret is not disclosable beyond current edition of Popular Science. TS requires reading good ‘net sites! :)