input welcome please

so me and my partner were driving home from a day of shopping on saturday. And out of the blue he says if you look at these girls walking around the city which do you think you would like to sleep with us, now that I was hesitant to answer, he knew when we got together 4 years ago that I liked women and had been with them before i got with him, but he didn't like the thought of me being with another while with him. I looked at him as to say are you nuts, he said to have a look, so i did i looked around and said I like her or her or she has a nice ass for sure, he then proceeded to say so if i said about me and you sleeping with a girl would you be interested. Now when he says that I don't know what to thing, I don't know if this is a scheming plan, but he has been dead set on it since, so basically he wants a threesome with me and a single female, he doesn't want a couple there because he doesn't like the thought of another male there with me, I'm confused becuase I was happy to go through with it and not today 24 hours on I'm kind of feeling negative about it, like I just shrug it off and put it to the back of my head when he wants me to look for a single female instead. we have a lady coming on Saturday night to have a chat and see were things go, but i just feel like I can't be bothered for it, I want to keep him happy and do this yes but why all of a sudden is he wanting this when it hasn't come around in 4 years, not to mention we are pregnant with our 3rd child. any input welcome please and thank you.