PLEASE HELP: My man wants me to invite another woman into our bed

I've been married for just two years with a 17 months old baby. My husband has been pressuring me to invite another woman into our bedroom for a threesome to spice up our sex life. Unfortunately we have two married friends who do this and he now wants us to include that element in our marriage. I am curious about it after hearing of it from our friends who indulge in it but I'm not comfortable with going along with it. More after the cut...

When I asked one of the women why she agreed to it, she said she would rather have her man cheat in front of her than behind her back like most husbands do. I don't want him to cheat on me at all but he keeps insisting on this and I don't know how to talk him out of it. Should I agree? I don't want him cheating on me. Please help!