Nope, you read it wrong. The ability to make her naked during Yoga unlocks AFTER the Sex scene after the bathroom.
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In the "Afternoon" part of the day search the top part of the drawer you put the sleeping pills into to get the 2nd yoga outfit, you will unlock the ability to have her naked during Yoga by performing the "Sex" scene (below) with Martha.
Sex; After you get the blowjob from Martha you unlock the ability to ask Martha about sex during your bathroom hand job, she will teleport to her room you must then enter and speak with her to initiate the scene. You are able to ask for sex multiple times but are required to get the hand job first. "
This doesn't mean "Yoga Sex Scene", but rather unlock the option of having her naked during Yoga