Thinking of starting swinging

my fiance and i have always wanted to have a threesome with another guy and the other day we took one step further from talking and she dirty texted and sent pics to one of her guy friends while we were doing it.  she has slept with this guy once and i can tell she wants to do it again. i am just worried that she will leave me for him. how do swingers seperate sex with other people from their love for one another?  also she would be very jealous if i slept with another girl but i am ok with her sleeping with other guys. does anyone else have this situation and does it cause problems with swinging? i dont necessarily want to sleep with another girl but i feel like most situations would be another couple and the other girl would have to sit on the sidelines which wouldnt be fun for her.