It's completely different for every relationship. Some poly people in primary partnerships find that they derive a lot of side benefits -- better communication, better trust, better sex -- from having OSOs. Others find the EXACT opposite, and find that everything is harder. Yet others find that there's relatively little impact.
In the end, though, what matters is how *you* are treated by your partner, and how satisfied you are with this particular relationship.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.