rowan naive I certainly am..............!!
You live with an ex partner, but you also have a partner.
Surely you ex, only becomes an ex when he walks out of the door for good.
To me an ex is when you both agree a relationship is over.... you may sleep together every now and then for whatever reason, you may even still live together, but your relationship is definitely finito.....
It can happen for financial reasons. When me and 1st wife split up we still lived in the same house together for a year after as neither of us could afford to move out. We both had partners after a while. It worked out quite well but not ideal. It was amicable enough for us to live separate lives under the same roof.