Atheism - have sex with whoever you want, when you want it. No right or wrong. Live together before marriage if you want. Have multiples partners. Practise safe sex.
What you've stated is something of a misnomer.
Christianity - God advises us to save sex for marriage.
So is this.
There are different "schools" and thought of Atheism just as there are different schools or thoughts of Christianity.
In the (Christian) Orthodox faith, for example, priests can (thankfully) marry, and their monostaries are renown for brewing brandy (spirits) and wine. As a result, pedophilia amongst men of the cloth is virtually unheard of.
Furthermore, sex before marriage in this branch of Christianity (one of the largest) is barely even mentioned. Sex is a natural human act comprable to breathing; longterm abstinence, it has been proven, results in a multitude of emotional disorders, among them phobias and sexual deviancies (such as the aforementioned pedophiliacs).
I do not know of any modern school of Atheism which touts wanton abandon for sexual intercourse, to the point which you describe as utter, indiscriminate acts. On the contrary, a vast majority of Atheists view humans as animals which mate for life (like ducks or some eagles). True, the Atheist explosion during the Rennaissance era included a, perhaps questionable, sexual openess, but that does not imply sinful degeneracy, rather a natural acceptance for human nature. Whereas the Western, Roman Catholic side of Christianity, paritculary during the Victorian era, has been responsible for horrendous acts of brutality, including genital mutilation to prevent adolescents from natural, normal sexual maturity. It is this fear of the natural, human sexual behavior from a large portion of Christianity which has successfully stunted the development of human education and learning. In the Middle Ages, literally everything was considered a "sin", from using herbs as medicinal cures to even bathing one's self! More than half a millenia later, Colonial Christians were still paranoid of sex, to the point of psychotic obsessive compulsive behavior. Adolescent boys had their penises strapped with electric devises devised to shock the recipient upon erection (a perfectly natural male function). Similar equipment included spikes which stabbed into the male genitals upon erection. These were beliefs and practises used within the last one hundred years!
...and I'm supposed to find Atheism shameful and promoting of terror?
I've yet to see any largescale open propoganda for Evolution being used for inhumane purposes. I could write an entire novel, however, for Christianity's terrorist accomplishments. Again, that's not to say Christianity, in itself, is "evil", but it is irrifutable that this religion has very openly used by the powers-that-be as a tool for terror and desensitisation.
Having said all that, I respect Christians and encourage them to their beliefs, but I ask for the Atheists and Evolutionists in the world to receive the same respect. Question one's beliefs, if you do not believe in them, yourself, but do not degrade them. Talking about the histories of various beliefs is one thing, making statements such as "I think <Evolution/Christianity/Etc.> is the most absurd thing I've ever heard of" is insulting the belief of some.