Original Sin

They’re just another set of human beings… you are not contractually or legally obliged to be nice to them, but in all actuality you’d better be…

I can write about this with some authority cos although I don’t have any at this time, I bloody well have had in the past.
They’re the parents of your live-in lover; what may be viewed in the eyes of 'god' as your sinlaws.
So, you‘re living in sin...
You would hope that this delicious buffet of sin lets you off the hook as far as familial duties with your other half’s family go, just leaving all the fun and sexy stuff with none of the responsibility! Waaaay! You get to avoid all the rubbish stuff like having to do paperwork if you break up (breaks-ups suck bad enough as it is without having to fill out documents).
But somehow, being as we are human beings, and therefore generally disposed towards harmony (at least most of us fairly sane, or 'can pass as sane' ones) you find yourself in situations with these people, dealing with the strain of hyper-politeness, trying not to swear, generally being on best behaviour, stifling all the best and most exciting parts of who you are and acting like you are some sort of a wholesome, mature individual who was certainly not completely off your face and naked with their son the previous night.

Part of the problem is that you subconsciously know that without that marriage contract, you're on shaky ground anyway...
Also, there's always going to be the thought that you just can't chase away that all they are thinking when the look at you is "you do nasty, dirty things with my boy"...

I remember biting my tongue when now ex-sinlaws came over for live-in lover's birthday. I slaved frantically and fearfully away cooking a roast and making a massive and impressive cake covered in icing, candles, the whole bit... Upon arrival, mother-sinlaw announced she had brought a (not previously arranged) pudding. I thanked her and said that in actual fact I had made a cake but I'm sure we could have a little of both. To which she replied in her trademark blunt way "You can have the cake tomorrow night." I sat through dinner (while every bite was scrutinised by said mother-sinlaw, all the while damning it with faint praise) then after they had left and we could barely move for gluttony, I stubbornly got the cake out and insisted on lighting the candles and singing the damn song.

All in all though, I've had a fairly easy ride with the sin-laws... If you're lucky they live in a different city. If you're even luckier, a different country. You also want to hope for sin-laws that have extremely full and busy lives themselves so there is no time or desire for meddling, with the added bonus that conversation topics are more plentiful.

That particular mother-sinlaw (of the unrequested pudding) is soon to be another girl's actual 'in-law'...
I can't help thinking I dodged a bullet there...