The condition in URM is "kari.status !=girlfriend and girlcount == 0 and status.index( kari.status ) < 1 and not monogamous". From what i can understand Kari needs to be my girlfriend and we are in a non monogamous relationship. I meet these requirements but i have no idea what girlcount == 0 means. It could very well be old code and i am just misremembering being able to pick a mistress option at that point.
I'm just about to have dinner, let me start a new game (Always have a save habdy early game for stuff like this) and I'll check myself and see if can figure it from all my saves and post back in an hour or so.
What version are you on and are you using one of the mods?
Unmodded latest version and no option for ir, not even hidden:
I can't even find a status.index in the code.
Girlcount is also an interesting one, sleeping with Ashley and Kari, girl count stays at '0' until you get the converstion with Lois in the office just before the trip, then it suddenly jumps to #2' for no apparent reason *shrugs*. I've noticed that variable being weird in the past though.
However, changing to 0, monogamous flag set to False and Kari Status as 'Girlfriend' (so all your variables bar one which I don't even know what it is) gives me no options (just as I expected). Same if sleep with no one except Kari.
Also, checking the Ch39 script, absolutely nothing in there about Lois being a Mistress...going to guess you're using a mod, or something has gone very wrong with your download/installation as not just not seeing here, neither in game or script itself. A new clean install might be a wise idea.