You're totally going to laugh about this years down the road ;)
It's not like your daughter doesn't already know that her parents 'do it'... she's here, right?
My middle child walked in on us one time. We had locked the door, but in our 'moment' didn't close the door all the way! Thankfully, she was sleepwalking and remembered *nothing*
One time I grabbed my guy and pulled him into the kitchen and gave him a big old fat kiss just because... my 7 year old came hauling around the corner and stopped dead in her tracks and yelled 'oh my god, are you guys making the sex?!'..... HAHAHAHA!! I was like 'no honey, not even close'...
My girls are 5 and 7. I've already, sort of, had the talk with them. My thing is, I go into extreme medical detail, they get bored, they leave. I still remember, as a child, kind of putting 2 and 2 together as far as sex goes at school from hearing other kids talk about it. My parents never had the talk with me... until I was 21, had 2 kids, and I was going camping with my boyfriend (who is now my fiance) and my dad threw a box of condoms at him. (My fiance laughed and threw them back at my dad and says 'oh, those will never fit, way too small'.... WOW, MORTIFYING.)
Anyway, lighten up :) Have the talk with her anyway. Neither you nor she is damaged by this, and I promise it will be okay. She didn't see anything. She may wonder what was going on and ask about it; tell her whatever you're comfortable with her knowing.
And remember, you WILL laugh about this later on :)