Sex is bad! Sex is Evil! Avoid sex at all costs!
If you want to know why the world perceives Christians as being anti-sex, just spent a bit of time in this sub.
Christians view sexual sin as the most sinful of all sins. We pile on people having struggles with porn and tell them how evil it is, and how evil people who use porn are. Many Christians consider looking at pictures of attractive fully clothes women as evil also.
Meanwhile, when young Christian couples are struggling with sexual issues, or if older Christians are struggling in a sexless marriage, they get a pat on the head and told to "pray harder," but don't talk about it. Because talking about sex with someone you are not married to is evil.
I grew up in a Baptist church. We were at church at least three times every week and I went to christian schools. I now serve on my church's tech team, so I am there every week. Every single time the subject of sex has come up, both in the churches I attended, and at the christian school I went to, it was within the context of "sexual sin" .... no church leader in my experience has said anything good or positive about sex. In fact, my youth group leader told me that "sex is always sinful, but God looks the other way if the couple is married." Literally. Those were his words in response to me saying to him that since sex is so awful, I am surprised that Baptist even have children.
So don't be surprised when your non Christian friends view Christians as anti-sex, because we are anti-sex. And we have been since long before I was born.