I believe it wasn't about adsense, I have seen porno ads instead of usual banner of my website couple of times when using internet in public places like libraries, in public places they usually use proxy servers and I think proxy servers are messing things up sometimes

the guy who have seen the gay ad might be using some sort of proxy server or maybe he is behind of transparent proxy (forced by ISP)

I'm sure Google wouldn't approve such ads in the first place, they are really picky