Are we talking like threesome sex with others, where you're there the whole time, or the kind where you get home from work at 6:00 and "The house is dark and the pots are cold" and she's been "hangin 'round with Tina Joan and a who knows who" and she comes back at 12:00 smelling of strange semen?
Honestly I'm in sort of a bad position here because I've done the second one to her before! I know I know I'm a horrible person. She totally knew about it, and I guess she thought it wouldn't bother her that much but afterward she got pretty upset...I feel horrible about it, and it's now just one of those things that you look back on and think "uugggghh" about yourself.
In conclusion, because of our unique situation, I would have to let her, but I'd probably be really jealous too, but I'd have to swallow my jealousy.