With the recent chapter in Confessing It, does Bill know that Dipper's "flirtation" by demon standards makes him an oblivious demon magnet? You did say had the events of Faking It never occurred, Dipper would strangely attract demons whether he likes it or not.
Bill is more than a little aware of what a nice human he’s bagged, or he wouldn’t have hooked up with Dipper in the first place!
Or rather, let Dipper hook up with him. After all, Bill’s not the one who went below the belt first. He was being a gentleman.
It’s also safe to say that Dipper, if the events of Faking It had never happened - would have had a demon go ‘Nice,’ right before keeping on with whatever they were doing. A demon who took a closer look at Dipper would have gone ‘NICE’ - But no way would they put up with Dipper’s defiant shenanigans, not once it stopped being Interesting and started being Annoying.
Bill was aware that once you get a good demonic look at his human (Which Dipper made Bill WORK to figure out, that tease) he’s a piece of eye candy.
He was not expecting what ended up happening. Not the first time Bill’s been blindsided, and it’s not going to be the last.