I have known for quite awhile that using a strap on or some version of exploring using my partner's ass would be pleasurable for me.  As I am exploring my boy's pleasure by using plugs and my hand on his ass....building to more...looking forward to more, even so I am amazed by how much of a turn on it is for ME.  Again, I knew I would enjoy it, but getting as wet as I do just from this activity....and discipline activities too...that level of arousal without yet being touched for my pleasure is beyond my expectation.