In that setting, late teens at a party, the only one way to find out if she really means “no” when she says “no” is to make her say it a couple times. Girls say “no” once or twice all the time when they really mean “yes, let’s do it, but I don’t want to feel like I’m a slut for wanting to”.
That’s just a fact of life. Or, at least, it was back then.