Traditionally, Christians believed the laity (not clergy) should marry to fulfill God’s plan, especially women. Bearing children, it was believed, was God’s plan and will for women. Given the very large and prolific Christian population worldwide, I am not sure why you think Christians prohibit male-female relationships. Most Christians today is the descendants of centuries of Christian ancestors.
In case you mean friendship relationships, some Christians fear contamination if they socialize with nonChristians. They fear being drawn into activities, speech, beliefs, and ideas that are nonChristian. Having their faith challenged feels dangerous on a mortal level. Why? They fear they are not strong enough to resist. If they fail their God, they know they will be separated from God forever after death, which they believe is the worst possible thing that could happen.