DinosaurPirate does an incredible job at explaining affirmative consent, for folks who still may have a problem getting it. She compares it to serving someone tea. Well worth the read.
If you say "hey, would you like a cup of tea?" and they um and ahh and say, "I'm not really sure…" then you can make them a cup of tea or not, but be aware that they might not drink it, and if they don't drink it then – this is the important bit – don't make them drink it. You can't blame them for you going to the effort of making the tea on the off-chance they wanted it; you just have to deal with them not drinking it. Just because you made it doesn't mean you are entitled to watch them drink it.
