I remember walking up to my parent's doorway, seeing that the door was cracked open a little and the light was on, and seeing my parents in an intimate moment. I didn't understand what I saw at the time, backed away quietly, and went back to my room. My parents didn't know that I'd ever seen anything. I was somewhere around 9 years old. That I remember it so vividly tells me that it made a definite impression on me. That I never talked to my parents about it says more about the nature of our relationship than anything else.
I remember one of my young teens walking in on my ex-husband and myself once. I honestly don't remember which one. Anyway, I remember how embarrassed I was, but felt I needed to talk about it with that child. In the morning, the child was clearly embarrassed and uncomfortable as well. She was, for some reason, sitting on the top of the stairwell and I sat down beside her on the top of the stairs and said to the her, "Is there anything you need to talk to me about?"
She immediately said, "No!"
Then I said, "I'll answer any questions you have, but if you don't want to talk about it, that is fine, too. I just want you to know that I love your dad very much, and he loves me, and sex is one way married people express their love for each other."
Then she said, "I don't want to talk about it."
And I hugged her and said, "Okay, I love you." and I got up and went downstairs.
I don't remember it ever happening again, and I think that child must have warned the others about knocking at the door, even in the middle of the night, and we were much more careful about making sure we LOCKED the door from then on.
I think that its wonderful that you and your husband are still sweethearts, and at the end of the day, that you love each other should give your child more confidence and security than anything else. Yeah, it might skeev her out a little, as it is uncomfortable to think of your parents as anything but parents. But, I think if you approach it honestly and without apology (after all, you were in your own bed at 1:30 in the morning) it will work out.
Hope that helps,
L.