Yes, I know, coming from me who is blunt to those who hire me to edit, telling people not to be rude can seem like the pot calling the kettle black. However, I may be abrupt in my comments about your manuscript, but I do it in a loving way because I want to improve your writing. The same way I yell at my children at the top of my lungs as if they just broke and antique or brought a dead animal into my house only to give them a hug.
However, there is no reason to be rude to a beta reader just because he or she does not want to read your book. The rude person who prompted me to write this post implied that a beta reader was like someone who was unemployed looking for a job and that his/her book was basically a gift to the beta reader.
Now, some books are very wonderful to read. However, any book in its beginning stages is kind of like a newborn baby before it has been cleaned off - only a mother (or the books author) would find it attractive enough to take into her (or his) bosom.
When I have finished working on a book, the book is inevitable better than before I start it, but I am getting paid. I can afford to put two or three weeks into helping you smooth out the wrinkles. However, a beta reader is doing it for free (dare I say "fun" in an S&M sort of way) and trying to find the gem under the dirt (frequently, they do not get it after it has been polished by a professional editor).
I would like to think that my manuscripts are a little nicer when they go to a beta reader since I do work in the profession. I am not bragging, I actually have had beta readers tell me they just needed a little more tweaking and were in good shape. However, I would not insult my beta readers into reading a manuscript for me. I am trying to send Radiography and Chemotherapy to a beta reader as well as The Lost Histories Book One. Both of these books have only been through one of my rough edits and I usually like to do a minimum of three before sending them off to anyone. I would in no way complain if a beta reader flat out refused to read these because of it. I am hoping, though. (Or maybe I should say groveling.)
Anyway, there are many beta readers out there. Find one that reads what you are writing and do not ask him or her to read something that is outside their genre. If a beta reader turns you down for any reason, even after you send your manuscript - thank them and be nice. They are volunteers, you know.