That was probably the first pro-gay mainstream movie, showing the conservative gay senator as an idealistic, admirable young man blackmailed by a liberal colleague. And, yes, at that time, in the 1950s, the leftists were anti gay (homosexuality was social decadence: for more on which, see "Spartacus.") The conservatives were not (gentlemen do not meddle in each other's affairs). (The same went for abortion, by the way.)

I'd hardly describe Advise and Consent as a pro-gay film--it makes homosexuality seem creepy and deviant and it depicts a gay bar, to paraphrase Pauline Kael, as one of the Seven Circles of Hell (any pick-up joint is, in fact, a Circle of Hell, but I doubt that Otto Preminger and his colleagues would have gotten so wracked up over a singles bar). It is true, of course, that liberals were as prejudiced against homosexuals in general as conservatives, and the Hard Left thought of homosexuality as some sort of Capitalist Disease (think of the novel and the film The Conformist).

But the one who seemed the most creepy and deviant was the liberal senator!

I assume you're referring to the character played by George Grizzard. What he seemed was nasty and ambitious and amoral. Not creepy and deviant.