I had always assumed I'd go back to work as soon as my children were at school - I was a librarian before I had DS, running a small branch in SW London, and when DD went to school I looked at library jobs, but in my area, all library staff had to work 3 Saturdays out of 4, which I wasn't prepared to do - also they were starting the long process of cutting down on qualified staff, so it might have been difficult to get a job anyway.
I started thinking about retraining, but in the meantime I was volunteering regularly at DS & DD's schools, as well as a lot of PTA work. Anyway I started a voluntary project with the chair of Governors, reorganising the Junior school library & by the time DD started there I was 'working' there about 12 hours a week. After a while, a paid support job came up which was 8 hours a week & I was asked if I wanted to do it - a bit later on they discovered I knew a bit about computers & one thing led to another & I now work fulltime in term time as ICT support manager - something I would never have remotely considered as a career, never having thought of myself as a technical person.
It has been useful being around in holiday time and after school - like a previous poster I would not have been too happy with the idea of leaving two teenagers in the house - not because they would have been alone, but because they wouldn't!
I don't think it ever really occurred to us that I was 'living off' DH or not making a contribution - when DS & DD are through with university, we are thinking of DH giving up his job to carry out various projects & I'd be quite happy to be the only earner if necessary.