Should I tell my parents about this new love problem?

My mom and dad divorced about 7 months ago, and my mom is already remarried to a nice, caring guy. He's all that I wanted a dad to be and he also has a son who is my age. One problem, though. Before any of the marriage things began, I was this boy's girlfriend. We had been dating secretly for 3 years, and neither my mom or his dad knew. But even though they got married, we have still continued being "lovers." I just feel guilty about everything because I usually tell my mom the truth. It's so hard to hide our feelings from our parents, but the whole marriage was just so unexpected and not fair to us. How could we break it to the parents? Or should we not tell at all..