I try to keep this as simple as can be and true to being as honest as I can.
I will not lie and be deceptive on something like this. I rather would separate first. I would rather be clear first that obviously the relationship tanked or is tanking and unless it is repaired, it is over. For the benefit of both. I treat her as I want to be treated. I like to sleep well at night, I like to know I did the best I could do to be an enlightened person, of light and not darkness as best as I can be. Not spin more suffering. Maybe I am a closet buddhist. So I don't see an SP (new world to me) or say find a FWB, ex-girlfriend (hmm which one), date incognito, or whatever sexual relationship without the other person I said I cared about knowing it is clear this is happening.
...and I know this because I am living it now. Been there, done that.
I do realize though, with kids and financial issues, this gets sticky for a lot of people. However regarding the kids aspect, people are often short sighted. Growing up I knew two friends who's parents hated each other and should have divorced, but stayed together "for the kids". Worst home environment of tension, rolling of eyes, fights, anger, bad talking the other to kids, etc. No display of affections to each other. It is damaging. Separating and finding new love and living that way and healthy is in my opinion better, and shows kids there is life even through trauma. So many people just don't see that.