I think your reaction will have confused and upset him and possibly even made him worry for you. I know how hard it is to be there for someone when they are upset on long distance. Take it from his point of view - you asked him to be honest and he was, he told you about a past relationship and then you disappear with no explanation. What if he had done this to you?

Now, poly relationships can tend to cause strong reactions in many people. Clearly you do not like them and that is understandable. However, you should probably have at least waited to establish what his current relationship status is and made your own position on the issue of polyamory clear. If not straight away (yeah, I can understand your visceral reaction to walk away and cry and no blame for that) then certainly after a short reflection time during which you compose yourself and think of what you want to know.

When starting a BDSM relationship there is always going to be a lot of negotiation and possible misunderstandings. The negotiation is important so as to avoid the misunderstandings as much as possible. If you make your position on other subs clear at the outset then he can tell you his desires in that area. If they are compatible, fine, if they are not you can decide if you stay or not. The important thing is that from both your points of view, everything has been settled in a sensible and mature manner.