Umm no where in Christianity that says Sex is evil. And their is a heck of alot more to experience life than just getting laid. Jesus didn't came down to earth to "experience" the thrills in life, he came down to suffer. To bleed and die on a cross, for my sins and the sins of the world.
EDIT: Suffering does Not necissaruly equal masocism. He didn't come down to enjoy the pain either. ie. In the Garden before Jesus was crucified he prayed. "Father take this cup from me.... but not my will your be done" (Not an exact quote, but basically hes saying, if their could another way, let me not have to do this)
Marriage is their to protect both parties, and protect the helpless life that can be created because of sex. The experience of sex, is increased when two people come together with full trust in each other and a desire to please each other, as is the design of marriage.
Now that it has been said. This is a gameThe last time I checked their was no Christianity as a choosable religons. In non of the Patheons is their a god with the name of GOD or Jesus. So basically its up to whatever the moral view in the actual world that has been created. To me seeing as their is no "Christianity" persay, then if both parties carried about each other, and your not rushing somone (like sleeping with Arie to early) then I do not believe theirs a problem.
Its a matter of respect in the person who gets to do the choosing.
Weather Virture should touch this or not, is really up to the discresion of its author. It would see acts of Rape as evil. Personally I think this shouldn't be touched as everyone comes at this with their own moral values, and as I said. This is a game, not real life.