From the Companion Guide:
How Do I get Desire Yoga Session with Extras Desire must be in the Gym alone in skimpy bikini with high stats. Two part achievement. First is easy just ask to do yoga with her. Next time under same conditions as stated above. Do Yoga again and this time she will break you off some Ooo La La yoga mat action then invite you to shower with her. Sorry you ain’t getting a second round, but you’ll get a sexy story from Desire as you shower. Look out for her Easter Egg trophy during this Scene. Save before it starts so again you miss it you can go back.