Don’t be subtle. Be direct and frank. “Dad, I don’t want to hear you having sex with your new girlfriend and I shouldn’t have to put up with it twice a day. Either be quiet or do it when I’m not home.” Throw in some imitations of him or her. He should be mortified enough to behave. Better yet, tell it to both of them face to face. That way there can be no confusion between all parties.
So IRC is really cool huh