I have a theory.
I know my dick wakes up before I do. Therefore, I believe my dick goes to sleep before I do.
A while ago we were at a party. It was a VERY long party. I was with a woman around 10pm and we got interrupted. Around 2:30 she wanted to finish what we started. I was all for it! We started going to town and I was nice and hard. I wanted to give her an earth-shattering orgasm. So I am licking her, fingering her, kissing her. She has an orgasm that makes her lose her knees and her legs go to jello. I reach down to put my dick in her. WTF!@#! My dick was not hard. My dick had decided it was time to go to sleep. I told her, "Well, this is interesting. I think my dick went to sleep. I think the clock struck 3 and it said goodnight!" She put my dick in her mouth to get me hard. Nothing. She tried every trick she knew. Nothing. I told her, well, this is slightly embarrassing, but I will make the best of it. She looked at me confused. I told her that I feel she had one more of those awesome orgasms in her and I was going to spend the time to find it and get it out of her. She chuckled. I spent 45 minutes with her and found it! I got it out of her. Now she was dead to the world! I told her that she should get some sleep, I will get some sleep and we will work on the dick in pussy thing another day.
So be honest. Don't get hung up on it. Make her want to fuck you later.