"to shaker" was used during my study time in GB as hot flirting. I asked once how the expression comes. I was told that there is a hot couple dance. *shrug*
Don't make conclusions about sex quality based on dudes size, and not even based on his dicks size. Brain is the biggest sexual organ, and if he can tease her phantasy, they have hot sex.
And I think, that it is nothing wrong in thinking about hot things in your situation
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And on the serious note...... You have to think about what is gonna to happen in your marriage. I mean sex is important part of a relationship. I don't want to cross here any personal boarders, but you started this and it makes no sense to keep your eyes closed and pretend that a problem is not existing. And I am pretty sure that your wife understands all that. You have to communicate somehow with her about the theme sex. And DON'T mention the hot yoga teacher!