As children get older, they start being more independent, ie will have sleepovers (we found that a good excuse for a night out or some 'us' time over a nice meal, watching film etc), be out with their friends in the evening and even going away for holidays with friends families. It starts slowly and you adjust as you go along.
As they get into late teenage years, they start getting more independent and even having ideas about how things should work in the house, so you can get tot the stage that, although, you love them, you're ready to let them go.
As they start getting more independent, yes you will spend more time alone with DH, but that's why it's important to keep in contact with friends and have hobbies. This week, I'll have done two exercise classes, DH has been on long walks and bike rides, I'm meeting a friend for coffee tomorrow and as it happens meeting up with two different couples on Friday and Saturday who we've got to know. It gives you different company, something to plan/look forward to and gets you out. DH is now retired, I work full-time. Although, we're in the house, we're not constantly together as we both have things we need to do, enjoy doing independently.