When you first start dating someone, you don't know who else they're seeing. For those first few months, both may be seeing others, but neither will say anything. They will make every effort not to be found out. It doesn't gain you points with someone to tell them you are sleeping with someone else. So, for me, I knew it was probably, I didn't ask, and didn't tell. Then, if we really liked each other, the relationship deepened, and we started spending so much time together, that I KNEW there couldn't be anyone else. Then starts commitment talk.