Clearly, this disease is purely a plot device. (like, duh!)
Because, if it really was a thing tied into endorphin (or some such) release due to arousal (which is the only excuse for increasing effect as it becomes more immoral!), then it wouldn't actually be male massage being effective, but rather massage by a person of the prefered sex.
It might be that it really is what I just speculated, but 1) the medical researchers didn't consider it a possibility, and thus didn't check to see if it was true, or 2) the medical researchers didn't have the proper test subjects to determine it.
The author really missed a bet when he didn't think of that; then he could have had her be bi, and have a couple massage her at the same time.
Or, make it yuri.
I guess he wasn't aiming at those audiences. If someone were to make a doujinshi... these ideas could be explored.