What am I afraid of?

Blue

My bf and I have been considering adding an addition to our relationship, just to try things out. He looks at the financial benefits of it. He says having another partner could bring in more money and we'd be able to save some Of course he's aware of the sexual aspect of it as well and so am I. I'm ok with the idea to a certain extent. I'm also a "curious" woman, I've had 2 past encounters (oral sex) with a woman before. He's expresses his love for me everyday and we are pretty solid even on our bad days. We don't want to be without each other. He expresses to me how it would always be me first and if at any time I got uncomfortable everything would be off.

So my question is, what am I so afraid of? It sound crazy but I rather be included in his fantasies then him doing it behind my back. I'm also currently pregnant, he says having a third person would really help us out. With our work schedules and school. Plus he has a 1 year old from his previous relationship. He's a great guy honestly. Imnkot quite understanding why I'm so scared.

Any of you in 3 person relationships. Have you ever tried one? Would you consider it?