Yeah, I always thought it sounded like a date rape situation to me. It's supposedly a classic song (it is a really old song, originally), and people (in those days) used to joke about a man slipping an aspirin into a girl's drink (the old fashioned rufi basically) to make her loopy. (It was called "slipped them a Mickey" - sometimes it would happen to men too, usually for robbery purposes.)
So yes it's a suspicious song - he's definitely trying to convince her, I also think it's coersion, and possibly leading to date rape. That's just my personal opinion though.
Also the phrase "candy is dandy but liquor is quicker" is a little sleazy in my opinion. I know it's socially acceptable to 'get a girl drunk' but, legally speaking, if a person's judgment is impaired by alcohol or drugs they cannot legally consent. I am not sure why more people don't realize that, though?
Anyway the "hey what's in this drink?" part...unless she's feeling woozy and he didn't actually put anything in her drink...is very suspicious.