Sex is sinful according to Christianity
When you say things so factually inaccurate, its either the result of great ignorance, or dogmatic bias..
There is a context in which sex is sinful, rape, for example, or sex with another mans wife, or sex outside of wedlock, but sex is actually something we cherish, something that God Himself tells us to do, "go forth and multiply".
In the NT we are told not to defraud our marital partner of sex. This directly refutes and puts to bed your claim that sex in Christianity is sinful, its simply not rational to say something like that.
1Co 7:2 But because of the temptation to sexual immorality, each man should have his own wife and each woman her own husband.
1Co 7:3 The
husband should give to his wife her conjugal rights, and likewise the wife to her husband.
1Co 7:4 For the wife does not have authority over her own body, but the husband does. Likewise the husband does not have authority over his own body, but the wife does.
1Co 7:5
Do not deprive one another, except perhaps by agreement for a limited time, that you may devote yourselves to prayer;
but then come together again, so that Satan may not tempt you because of your lack of self-control.
As you can see, the correct context for sex is not only biblical, but encouraged.