When trying to sort this out, many Christians have turned to these texts for guidance:
Genesis 1:28 (sex is for the God-intended purpose of pro-creation);
Gen 2:24 (two become one flesh in marriage - between a man and a woman, of course);
Hebrews 13:4 (purity in marriage);
1 Corinthians 6: 9-10 ("Fornicators, idolaters, adulterers, male prostitutes, sodomites," among others, will not inherit the kingdom);
Ephesians 5:3 (warning against "fornication and impurity";
Romans 1:24-27 (the "degrading passions" of exchanging "natural intercourse" for same-sex intercourse).
Our conversation will cover these texts, but we'll expand the scriptural exploration to these texts as well:
Luke 6:31 (the Golden Rule);
Romans 12:9 (genuine love means mutual affection);
Romans 13:10 (love does no wrong);
1 Corinthians 13:4-8 (love is...)
Ephesians 5:33 (love and respect between husband and wife)
Colossians 3:17 (do everything in the name of Jesus Christ)
Perhaps this will change the way we form our sexual ethic as Christians and give us a more meaningful application of that ethic in our contemporary society.
Some articles for additional reading:
New York Times Article that discusses research on best practices to Sex Education in the country.
One Christian perspective on the topic of Sexual Ethics
What the heck is an "ethic" in the first place?