I would like to remark that modern email clients do a very good job of protection from HTML mail, so that as long as one doesn't click on any element in the message, one can't normally be infected (with the infection vector being here thru the browser, and not the email client).
In the past, a remote image with parameters could have signaled the spammer the existence of your email account. Nowadays all major email clients retrieve remote images without parameters.
In the remote past, scripting was possible in email. Nowadays email clients flatly refuse to execute any sort of scripting under any condition.
As last comment, some emails might arrive as html-only. I also sometimes receive email where the text part only contains the message that html-viewing should be turned on. In addition, while viewing only the text part, you miss out on some purposely emphasized text, emoticons and other communications.
To answer your question, I must remark that I don't believe that Opera Mail has a text-only option.
You might need to migrate to some other email client for that.
I believe that Mozilla Thunderbird would be a good cross-platform solution
(from the View menu, select Message Body As, and then select Plain Text).