Personally, it is no worse than having a child. Billions of people have done it successfully, and (hopefully) billions will contine to do it for many many years to come. I think you just need to be ok with getting your "freak on", and if you get caught, then you get caught. Invite her to join or not. If not, politely say something like "Oh, sorry. We were just leaving. See you in a little while.". At least, this is how I would handle the situation if one of my kids walked in on my wife and I. (We have two kids. 18 and 13. We've been interupted a few times.)
There is a lid for every pot...Sometimes even two or three...