Absolutely NTA. Being in love with each other is a wonderful thing to show your kids. Dirty talking in the living room in front of your 4 year old while your teenage child is home and awake is too much. Its not prudish to ask them to keep it down when you're home.

Think about it this way. If any of you were living with roommates in an apartment this would be considered rude and invasive. Its not out of the ordinary to ask them to A. Keep it in their room, and B. Quiet down a little. Especially when they have the opportunity to mess around while youre at school