When you allow iMessage to mark messages as read, the person you are talking with is able to see when you viewed a message. This can be a handy feature, sure, but it also can make you look like you're avoiding someone, even if you're just busy and can't answer right away.
For that reason, I've disabled read receipts in iMessage. I don't like the guilt that comes along with knowing a message is marked as read, and the person on the other end thinking I am avoiding him or her.
As we've previously discussed, 3D Touch on the new iPhones enables two new modes of interaction called Peek and Pop. You can peek at something, say an email or text message, without actually opening it with a light press on your screen. A harder press Pops -- or opens -- the item.
So in order to view a message without opening it, lightly press on the conversation thread in the Messages app. By peeking at the conversation, you're able to view the latest messages without opening it, thus triggering a read receipt.
I've been using this to quickly view and reply to messages since the launch of the iPhone 6S by sliding up while peeking at a conversation and selecting one of the predefined replies, but never thought about it as a means to avoid letting someone know you've read a message.
If you find this to be too much work, you can always disable read receipts by launching the Settings app, selecting Messages, and turning the feature off.