How can i not be pregnant? Can someone please explain?

My bf and I are TTC for baby #2. We have sex almost every other day and he ALWAYS finishes inside me. I'm in my early 20's and he is in his late 20's. I don't have any fertility problems in the past or now that I know of and he is healthy as well. If we had sex almost all the time during this month and I am not on birth control, HOW is it possible that I'm not pregnant? Wouldn't I literally have to become pregnant if I release an egg every month and he's finishing inside me? I know this sounds stupid but HOW is it that I'm not pregnant? This is NOT meant to be offensive to any ladies with fertility problems or others who have not yet conceived. I am genuinely curious.