in one day.
Yoga moves can't be protected by copyright. As you're probably aware, we wrote about a similar situation last year in which the so-called "owners" of Bikram Yoga techniques sued 'Yoga To The People.' We're not aware of a resolution of that dispute but shortly after posting our response, the Copyright Office weighed in with a letter stating that yoga moves, and the ordering and selection of public domain yoga exercises, are not copyrightable. This letter does not have the effect of law but the opinion conforms with most copyright experts -- physical practice techniques are not subject to copyright. Therefore, we don't believe the orgasm techniques you borrowed or invented will acquire copyright protection (though of course original illustrations or text about those techniques can be protected). As for trademarks, it's possible that, like the Bikram folks, someone can own trademark rights to the name under which a physical technique program operates -- for example "Bikram Yoga" or "Bikram's Yoga College of India." But it would be difficult, if not impossible to acquire trademark rights to the name of a specific exercise. Nevertheless, if you're paranoid, change the wording of exercises you borrow, to avoid any confusion. (BTW, patent protection for novel, nonobvious techniques is always possible ... although we have to wonder, are any sexual techniques nonobvious?)
Contracts and permission. Yes, the email should suffice as permission with a couple of caveats. (1) Even if a technique cannot be copyrighted, a person can contractually agree not to copy it. (2) Some contracts prohibit modifications unless there is a writing signed by both parties. We doubt whether this will be an issue for you and we understand the need for informality so you're fine as is. Nevertheless, if you are concerned about formalities, you may want to amend your contract to provide for the permission.