In Thunderbird I have such option and I use it. Now I am trying to switch to GMail and I can't find such setting.
I claim that no computer program can know, whether I have actually read a message. This is because there is no implicit feedback from my brain to the computer. Thus, automatic marking as read just because a message was displayed is a serious mistake in my opinion.
Unread emails are important for me, because I must be sure that I do not accidentally miss an important message. Sometimes I even read a message and leave it as unread to know, that I have to read it again (e.g. because I do not understand something and have to check it).
How do you deal with this issue? How are you sure that you have dealt with everything important in your emails?