What do you mean exactly? If you've setup the email to have both text and html, and a user is only viewing text, there should be no issue.
I'm not sure why Marketo would re-send an email in any case other than a temporary server issue on the target's side.
I want to resend emails to users who haven't opened their emails.
Text emails are tracked so a program will probably resend emails because the status won't change to opened.
If you knew which of the two versions of the email that were sent (text vs. HTML) were actually delivered to the user by their server, you could theoretically not resend those that received text emails. However, I don't think it is possible to know this. Even if you did know, Opens are not a reliable indicator that someone did or did not open your email. Most marketers only send a follow-up version of the email when the recipient didn't click or respond to the initial version.