Interesting
My view
The title of the thread perhaps indicates The OP view on this.
Yes I think that is a fair comment, I think in the interests of being modern we have pushed it that way. I don't think in pushing it that way we have actually gained anything.
Looks like a father/daughter conflict
No not at all, its a father stepson thing with one eye on where my daughter comes in all this and a realisation that the girlfriend is someones daughter.
I do wrestle with this for many reasons perhaps I think too much but where do you draw the line. How do I now tell the 15 year old daughter that "no your boyfriend is not sleeping with you" when she see's the 17 year old boy doing so. At what age is it ok, why is it ok, how long is a serious relationship, what makes it right?. They will do it anyway isn't in my opinion a reason to say fine do it here. What next jumping into the jacuzi together, whats your reason to say no to that if you say yes to the other? it seems I am expected to say fine treat my house like a hotel and have sex when ever and where ever you like.
Perhaps I am being naive but my thought process is the more difficult and uncomfortable the venue the less they will play, this then aids the independence gene ensuring that they build their own nest to play in

I know that isn't how it is but I know it was one of the factors in me making my move away from home. I also know that supplying issues with many shades of grey makes things esculate to places that things shouldn't. Keep things nice and simple, black and white, lessens confusion.
I don't know, I just know that the word Respect is something that is rapidly disappearing from our society in every area. I would not have ever dreamed of asking my parents it was just not on the agenda, yet I could talk openly to both of them about all aspects of sex with no embarrassment at all.