I agree with everyone else that your dad is really stepping WAY out of bounds telling you about his sex life - that's really really unfair to you.
On the other hand, your mother knows about this. It's not a secret to her. He's just decided to act like it's some big secret - but she knows and is dealing with it however she is.
I agree that it is unfair for your dad to put you in this position. Do not get involved, however, with your parent's marriage. He's leaving her most likely. What you can do? Tell your dad that he tells her the truth or you'll make sure she accidentally finds out. (as chesh chesh said).
Try to stay neutral. But who am I kidding. My parents divorced and I sided with my mother. She was in the right. She deserved my support. I still maintained a relationship with my dad but my mother had my heart. I gave her first pick of holiday time (where I spent it), did not cozy up with my dad's new "chickie", and did not participate in any drama that ensued from the divorce. I wasn't rude to anyone but had my priorities in line for where my attention went.
So, stay neutral but be there for your mom is what I am saying. Sorry this is happening to your family
I dont think your father should be putting you in this awful situation,this is his marriage and his problem,if your parents are seperating then there is going to be alot of hurt that comes with it,but to tell you things and then your not aloud to tell your mother is going to make things ten times worse,your mother will think you are taking sides and that you are lying for your father,this is not a position that your father should be putting you in,tell your dad to man up be honest because you are not going to keep his secrets from your mother any more.
I understand you are an adult and such but why is your dad sharing all of the details of this with you? Does he not give a sh!t that she is your mom whom he made a vow ( regaurdless of the reason) to honor her as his wife.... And you as his daughter dont need to know how and when he is going to be breaking said vow.... I dont know but firstly i wouldnt want to know if my dad were hookin up with someone else before the divorce were final. Tell your dad to stop because he is putting you in a horrible spot. IMO