Kind of feeling like the odd one out, but I will share nevertheless. The immediate reaction of my husband were the basic questions (as I had already someone in mind when telling him about my 'poly-ness'): Do you love him more, Do you want to leave me, What do you want from him, Do you still love me ... The last one was: What should I make of all of this?! Which I couldn't answer anymore.
After some minutes of quick processing, he was so relieved that something like falling in love again hadn't ruined our marriage (cheating, affair) or my feelings towards him (despite having a new love interest I was still in love with him or at least was claiming to be at that point in time from his point of view), that we naturally reconnected via sex that evening.
After this immediate reaction he needed roughly three weeks to wrap his mind around the new situation and many talks with my friend to come to the conclusion, that we should merge our households and my friend moved in. That's it for the immediate reaction. Yes, he later had some rough nights and days, constantly thinking about the changes that were bound to come, but I guess he did reasonably well in not loosing his mind over this.
After the first insecurities were gone, and after everything had become more normal, we once talked about the benefits he could see in the presence of my friend in our lifes. He said, that he liked this way of living (three adult household), the shared responsibilities, how our dynamic evolved with another person in the mix, how he liked talking about me to him and to a certain extend, he liked the thought of another love being in my life (added some sexual tension and excitement for him) as well as it was a good feeling for him to witness me being true to myself, loving freely while never excluding him in the process.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.