Of our six kids, the 4 y/o and almost 18 month old still sleep with us. Hubby and I still manage to find "us" time.
When all the kids were little we just locked the door and went about our business. As they got older (and there got to be more of them), we began scheduling in time for ourselves (kids visited Grandma and Grandpap, went to visit their friends....Just got locked out lol). Depressing, but functional.
Sometimes hubby (or I :) wanted a quickie, which is easily enough fixed by popping in a movie. Kids will flock to it--we don't have a lot of TV around here.
Our kids are 13, 11, 9, 7, 4 and 18 mos now and we just tell them plainly that Mommy and Daddy need time alone. We decided when the oldest was around 8 that we had had enough of "scheduling" sex, and wanted to be able to be more spontaneous. So we just explained to them that, just like they all wanted to spend time alone with me or Daddy, we wanted to spend time alone with each other. And that, for the two of us to be happy and be able to devote time and love to them, we needed to be able to HAVE that alone time.
Our 13 year old recently told his Dad that he was glad we had our "alone" time. Apparently he likes that he gets to "watch" his younger siblings and "be a man". He also told ME the other day that it made him happy that his Dad and I love each other so much. I thanked him and told him it made me happy, too, and he replied with, "So many of my friends think it's gross that their parents get alone time. I just think it shows how much you love each other, and us, too." I asked him what he meant and he said, "If you didn't love each other so much, you wouldn't have made us. And if you didn't make us, then you wouldn't love each other so much".
Not 100% sure about that logic, but it was great to hear my son approves! lol