Our kid walked in on us when we were having sex. Now what?
Once you have recovered from the initial embarrassment (or the initial shock!), and you are sure he or she witnessed "the act," talk with your child. With your spouse, take the time to find out what is going on in her head. A good way to start is with an open-ended statement: "So, I'd like to talk about the other night when you walked in and saw me and Dad." Don't expect anything but groans and eye-rolling, but persevere!
It's better to find out what your child is thinking and feeling than leave her to work it out alone. At that age, you've most likely had the facts of life talk already (and if you haven't, this is your perfect opportunity!) so you can let your child know that you and your spouse were showing each other love in one of the ways grown-ups do. You can add some humor: "You're actually really lucky to have parents who are still crazy about each other!"
This also is a great time to talk about boundaries, potentially also in a light-hearted way: "If you're not on fire or in pain (and no one else is either), when mom and dad's door is closed you need to knock first. That way we can make sure to spare you this embarrassment in the future."
Finally, don't let this incident scare you off from making love! Walking in on their parents making love is a cliché experience many kids have and still escape from unscarred. It's a true statement that your child is fortunate to have parents who love each other; it is one of the most important contributing factors to a child's emotional well-being and sense of security.