If McDonald's is shit then that proves how good they are at marketing
The best way for selling porn is clever preview videos. Not too long, not too short and very suggestive, don't show them the actual thing. And I agree with Damien; treat your customer with respect, give him the product you've promised him and don't give him anything else like ads etc.
The comparisment with McDonald's is a good one; someone is horny, sees a McDrive, knows what to expect from there and will go there.
Someone is horny, sees your site, bought your product before and knows what to expect from it, and will buy again.