I think the harder thing, rather than having the sex ruin the friendship, is finding people who don't automatically assume that sex = full intimate/sexual relationship/more than friendship.
I think more than people thinking sex will ruin friendship, they believe that it has become more than just friendship in the first place. Perhaps both go hand in hand.
It has become more than a friendship, therefore the friendship is dead.
But I don't think you can have that intimate relationship without a friendship. You need to be friends with your partner. You can't love someone until you've at least found that friendship level with someone.
I think as long as everyone just accepts that moment for what it is, just sex between some friends, you can stay as friends.
The only difference I see between a friend and a lover, would be the regularity of your encounters with each other.
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