Sex with someone else

So my boyfriend and I have been dating for a year and we have a beautiful 5 month old daughter together. Yesterday he asked me if he could find a girl that wanted to do things with me if I would be willing to. He wouldn’t be doing anything other than watching us and I’m really curious as to what it’s like to be with a girl but I don’t know if it will be bad for our relationship. Please no negative comments, just need some advice