Threesome

So my boyfriend and I were talking about opening our relationship and basically I would be okay with it if I would also be sleeping with other people but he would prefer if I didn't.. so I want to have a threesome with him and the girl who originally brought it up to us and she's down but he's reluctant because I'm bi and he's worried I'll like her better or fall for her.. Basically I'm with the only straight guy in the planet opposed to a threesome with another girl... Ugh