Replying to Girls friends are they off limits?

All I do is ask a girl out (she wanted to 'stay friends') and her friends (who are good friends with me) start trying to sleep with me, for some reason this was completely not what I thought would happen. I thought the girl would date me but nope and I am totally cool with it and her friends say she messed up and I ended up sleeping with this girls friend and her other friend also really likes me. I had to stop her twice before because I was seeing this girl but I gave in this time.

What do you guys do? Any of you can end up in a situation like this I randomly did some how I only wanted to date their friend and acted nice to all of them now they are all over me and say I their favorite to have around and they always ask me to come over and they both make out with each other when I am in the room.

My conclusion is no. But maybe I am fooling myself. All these girls are friends in some way which is the strange thing and they say I am better than all the other guys they know and they always tell me that.

Why do these unexpected turns in events happen? All I wanted to do was date this one girl and this is what happened it astonishes me how little I knew where this was going to end up I had no idea.