Why do Christians beleive that it is imoral for men to have sex with unmarried women?

When most of the Bible was written - it was assumed that a man having sex with an unmarried woman would be required to then marry her or risk defiling her for life. An unmarried woman who was not a virgin had little status or hope in society because she became tainted goods.

Not fair - but true.

EDIT: check out these BIble verses that deal directly with pre-marital sex:

Hebrews 13:4

1 Thessalonians 4:3-5

Exodus 22:16-17

Basically, I think that when the Bible talks about "sexual imorality", premarital sex is included. Why? Because the Bible also mentions that the point of sex is the physical intimacy shared by partners, which should not be shared with others. If you have sex with a lot of people - that is a lot of sharing. Back then, it was considered perverted to share a bed with many... and it was also a lot more physically dangerous then as they had no way to prevent diseases. You don't have to agree with it today - but a case in point would be Jesus' mother (Mary). Joseph was going to divorce her because he assumed she had had sex before marriage since she was pregnant. If it was totally ok to have pre-marital sex... then he wouldn't have cared, right?

Just adding a little historical context.

Paul in the Corinthian letter states that having sex is getting married. So that may be the way it was- but from Gods point of view from that point on they are married.

God hates divorce, that means that one or both parties fails to continue the relationship with all of the benefits and responsibilities. Causal liaisons fall into this area. One or both parties has no intention of it being permanent and that is why or how it is a sin.

The Bible says that no fornicator will enter the Kingdom of God.

God forbids us to commit fornication or sexual immorality whether we be married or unmarried. 1 Thessalonians 4:3 says to abstain or stay away from.

If she have a baby...or if she will be married and have a baby and doubt about his real father...or if his father discover that he is not his son...that is why religion forbid any sex relation out of marriage...to keep every things in its original place…

Often when we judge or philosophize with the words we mindfully construe (dualistically constructed), we actually tell others where we are. (This is not something we're always consciously aware of especially when pointing fingers)

because god says it is immoral

The Bible only forbids a man from having sex with a married women?

no it does not

god forbids single people from having sex with other single people

it does not authorize married men to have sex with single woman

The Bible says NOT to have sex before marriage. So men and women both CAN'T have sex before marriage. Please study the Bible properly before giving false news.

the same book says we can stone disobedient children. but then again Americans LOVE Kipling their own kids. it's practicallly an epidemic.

Lie much, atheist?