I've been doing this for years and don't feel the least bit bad about it, and I think everyone should adopt this practice.
If marking as spam or unsubscribing are approximately equal effort, I will unsubscribe; if unsubscribing is even slightly more difficult than marking as spam, I mark as spam with no regrets. Even if you don't force me to login but make me do some monkey trick like type "CANCEL" into a box => spam! One click is the only acceptable method, and don't hide the link in a bunch of small print legalese, because I'm not going to expend much effort looking for it while the nice, inviting "Mark as Spam" button is just sitting there waiting for me to click it.
If marking as spam or unsubscribing are approximately equal effort, I will unsubscribe; if unsubscribing is even slightly more difficult than marking as spam, I mark as spam with no regrets. Even if you don't force me to login but make me do some monkey trick like type "CANCEL" into a box => spam! One click is the only acceptable method, and don't hide the link in a bunch of small print legalese, because I'm not going to expend much effort looking for it while the nice, inviting "Mark as Spam" button is just sitting there waiting for me to click it.