Is it OK to have sex with a condom without being married?
Having sex outside of marriage is a sin. The old word for it is "fornication," though newer translations tend to say "sexual immorality" instead. "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God" (I Corinthians 6:9-10). It is the act of sex with another person that makes it wrong, not whether you get the other person pregnant or not. See: Why Sex Outside of Marriage is Wrong.
Though the world likes to called having sex with a condom "safe sex," it is really a misnomer. Condoms reduce the risk of spreading disease, but it does not eliminate the possibility of getting a sexually transmitted disease. Condoms reduce the odds that you get a girl pregnant, but they cannot make the odds zero. In fact, a married couple regularly having sex have an 85% chance of conceiving a child in one year. With condoms, those odds only drop down to 20%. There are a lot of reasons why, but suffice it to say that anytime you have sex with a girl, you are taking a risk of becoming a father as well as taking a risk of catching a disease. If you do father a child, you are responsible for the child's support until the child becomes an adult. Neither of these addresses the moral fact that sex only belongs in marriage, but I wanted you to see that even non-Christians have reasons not to put their penis where it doesn't belong.