It's not a contract for sex - it is a statement of interest by the guy. Guy buys a girl a drink, he's saying "I'm interested in you, and want to take the next 15 or so minutes to see if the interest is mutual." If the girl accepts, she doesn't owe him sex. She's telling him "take your best shot, and I'll see if I'm interested."
Now, a lot of women won't do even that -- they'll take a beer and casually lead on a guy just to get free drinks for a night. It's kind of a crappy thing for a woman to do, but it also is the guy's fault for being dumb enough to keep buying her drinks.
bingo
The two are not mutual exclusive. You would be hard pressed to find a situation were a man bought a drink for a woman that was not paid for by his loins.
Exactly right