When can the new girlfriend sleep over? 

I've been dating a great lady for 5 months and we've been sleeping together for 3 months.  They last month or so I've had her stay at my house.  I have my two teenagers every weekend and most other days as my exwife is rarely around.  Recently my 16 year old daughter heard us having sex and got really upset.  We weren't screaming or anything, just muted passion.   Now the ex knows and wants to recommend not having "sleepovers" until the kids (15 year old boy and 16 year old girl) are out of highschool.  I think it's absurd, but I'm all ears.  Of course we'll keep it down from now on, but I can't imagine not sleeping with the one I love for 3 more years so the kids don't think sex is happening? any advice...??? thanks,