Awake enough to catch a sexual reference someone makes. If someone makes a corny sex joke, and you don't catch it, and you're kind of sleepy, you'd be what we call "sexually asleep".
Chemistry Teacher: How many balls do you have?
Joey: 2 BIG ONES!
Chemistry Teacher: No, the correct answer is 12.
In this scenario, the teacher is sexually asleep. The opposite is being "sexually awake".
Being a horny teenage boy, I can't think of a time where I'm not sexually awake.