I know this isn't my place to get involved
you've got that right
1. she likes having sex with you
2. she likes having someone around when her husband is away
you are way down the totem pole here
She's married and you're handy as a sexual / social convenience. It is a classic **** buddy scenario. Nice people call it FWB.
If you're ok staying on as a convenience for her, so be it.
Be very very very careful at work. This is the kind of thing that gets people into big trouble.