Husband wants open relationship... again

Been with hubby 12 yrs, have 2 kids and most of the time we are great. We have settled into new routines because he got a new job 6 months ago and we both work different shifts ( we have since before my kids). He convinced me to have an open relationship before, about 7 years ago. Seeing as I was the only person he had sex with, I thought it would be ok to let him explore. We laid down rules, and both had sex with 3 people. I did not really get any satisfaction from it,and we stopped when we realized we spent more time talking to other people than we did with each other. Also the last women he was with fell in love and was ready to leave her relationship thinking he would do the same. Fast forward to now.. he has brought this subject up plenty of times and I've been straight with him, telling him how I don't want to be in that type of marriage, and that we should focus on us and our family. He brought the subject up one day recently and I didn't want to end up in a fight so I told him we would talk about it later. Well after several times of pushing it off, I finally said NO again last night when he asked if we could do a

The only thing a "Open Relationship" produces married peopled sleeping with others in plain sight. Both of you would have to be in agreement with this relationship. He seems to have no problem with this arrangement; since he's continuing to bring up the subject. Do you think he will sneak behind your back anyway? Who is he wanting to get with so bad to put his marriage and family jeopardy? Now that you have said no, what was it that he asked "If we could do a"??

@SKINNYGIRL2 I'm sorry it cut the rest of my post off. He asked if we could do a trial run, which I'm not even sure what that means