Ok they weren't in laws, but my ex gf's parents were having marriage issues and living separately, when their daughter and I were going out around 12 years ago. We must have been around 15 or 16.
So anyway we had both had lost our virginitys together that year. Her mother had a heart to heart with my girlfriend and news came out that the deed had been done...I had no idea this conversation took place. She basically had looked for some issue her husband and her could work on or whatever.
So one evening my gf and I are watching TV lounging on the sofa. Her Dad walks in and takes a seat. "Oh Hi!" My gf says as her Dad hadn't been living at the home at the time. He says nothing and is just staring at me. Her Mum walks in and turns the TV off.. we notice something is up.
"Its come to our attention.." her mum began. I'm already getting nervous at that phenomenonally formal start to the conversation. "..that the two of you have started having sex".
silence and blank staring
My GF darts out of her seat in tears "fuck you, how the hell could you embarrass me like this...etc etc". She leaves the room and slams the door, leaving me on the sofa alone, like a lost kitten in headlights. It was so tense that if I had stood up the sofa would have come with me.
Her Mum raised her eyebrows "Well we wanted to talk to you alone anyway..". Her Dad still silent and with a "Im going slit your" face staring directly at me. "So was it forced?" Her mum asked. Straight off the bat Im a rapist in their eyes. I of course assured them it was a mutual thing. "Did you use protection?" I told her that we used a condom and she was on the pill. She says this isnt enough (?!?!). That I need to use some gel or something as well, but that is irrelevant as we aren't going to have the opportunity for it to happen again apparently (it totally did happen again).
My GF bursts into the room screaming and shouting at her parents, i sit there awkward as fuck. I lived around a mile away, and once there was a silent period Im all like "well I best hit the ol' dusty trail..". "No you arent going anywhere, your parents are on their way".
After contemplating what object in the room would kill me quickest, in the most horrific silence that has ever been, there's a knock at the door and my Dad joins the party.
"So whats happened?", hes just been told there is an emergency and he has to come over quickly. They then explain the whole thing whilst my GF and I go to the next room and sit in the next room wanting to die. Dad sounds all responsible and everything, gives fake shocked responses and everything. We leave and my Dad and I get in his car.
We are silent, for around a minute. My Dad bursts into laughter, "Fucking hell, you got off lightly! My first time I got chased out of the house." Luckily my Dad being a rational understanding guy got what had happened, and just told me to play safe etc.
Tl:dr. GF reveals to her Mum that we had hooked up, mother and father then interrogate me thoroughly.