Here's a scenario for you. Your customer searches google for sex education and gets over 60 million results. Your domain sexforthebeginner.com comes in around position 30 million (on such a high traffic keyword, even blackhat techniques won't get you much further above number 100.
Imagine you owned sexeducation.com. You could quite literally do no SEO, have shed-loads of ads on there and still be ranked in the top 2-3