I've been married twice and been in the swinging lifestyle with them both (for years) during our entire marriage. Watching your wife/GF fuck another man, for me, is one of the hottest experiences you'll ever encounter. Seeing her the center of attention, writhing in the glorious painful pleasure of being taken by another man, hearing them both moan and grunt while they take each other is an absolutely amazing mind and load-blowing event. Further, being an orally bi male getting to suck him clean and/or eat his load out of her until she cums is an added bonus! For me seeing another man (or men) with a huge raging hardon(s) take my woman is a huge turn-on and something I will do for the rest of my life with my current girlfriend or the next or the next or the next...you get the idea. To take it further, sending her on "dates" only to come home full is also a huge turn-on and an entirely different subject but the next logical progression to "sharing" her.
Now, to your question: How to broach the subject? My suggestion is to watch porn and have A LOT, A LOT, A LOT, A LOT of foreplay! The key is to build into it and GO SLOW!!! There is a slew of sites that offer countless movies online that you can watch together. BUT! Have a game plan! Start with suggesting watching something "vanilla" (i.e. straight with one-on-one MF) while in the midst of foreplay kind of stumble into watching a threesome. Start watching and say nothing in the beginning. Just watch and gauge her reaction. If you have taken your time and built up to it when you finally start watching MFM, or even MMF or FFM videos she will be accepting. Just know going in that MOST females are not accepting to threesomes, especially FFM! Make subtle comments and suggestions. Let her know she'd be the "center of attention" and that it would be all about her at the moment being totally satisfied, pampered, and catered to. Probe her fantasies and desires at the same time but again GO SLOW! This process may take some time as in weeks and even months to flesh out. Express how you would feel, what you would want to see, and explore but be gentle and a little guarded as it will possibly get her to think and ask questions. If she asks questions, she is thinking!!! BUT don't PUSH!!! If you REALLY want to share her with another man/men then you have to make the entire thing center around her and make her feel like it is ALL about HER! Also, make sure you have your first encounter in a location where SHE will feel most comfortable. Most women will NOT want to be in their bed with another man and will prefer a hotel instead. Females will need to mentally remove themselves from "home" in order to "let go" and allow, enjoy, complete the event.
Now for the mental gymnastics for BOTH OF YOU! Be prepared to feel a bit "OFF" afterward. It is totally normal. You'll go from totally being turned on and wanting to do it all over again to full panic attacks and regret. ALL of it is totally normal!!! To this day I still have these emotions from time to time. BUT if you are secure in your relationship it will be a non-issue within time. NEVER refer to it as "swinging" or any other defining negative term. I usually refer to it as "PLAYTIME" or special event time. Also! YOU must remain in the power seat of control...this is a bit tedious and controversial...but "HE/THEY" are NEVER to have a direct line of communication with her!!! If she, or HE or THEM, wants to "PLAY" again then you are the one to set it up and control the EVENT otherwise you kick things into another category (another subject for another time, Google Cuckold) and things could possibly go south on YOU quickly. Remember, if you kick a sleeping lion... You might awaken something in her that you may not be prepared for...but it will be fun nonetheless!!!
Hope it helps!!!