When someone says that, it means that your not enough for him or her so they obviously want someone else to fill in what you don't have.... Which only runs back to u and is your choice if u want to get played but I were u I wudnt want to b put through that crap but that's all on you:)
Was he breaking up with you? Was he trying to get you into a three-some and wanted too get a rise out of you? Was he saying that he's not a good place to put all your eggs in one basket ( sorry overwhelmed with Easter eggs today) because he's got some playing to do.
Your lover seems to be bi-sexual. I'm not the first to say it. But, look, that's all that can be taken from this question. You guys need to have a talk, it seems. Because, you're not sure how serious or why. Maybe if they know it freaks you out, they'll back off the idea. Maybe, you wanna be involved (lots of couples swing like that). Maybe you don't wanna know about it.
The upshot is this. You asked "What does it mean...?" It means it's time to talk. Everybody's got an opinion about what to do. The only opinions that matter are the two of yours.