A study from CARA asked that question of Catholics and Protestants — and the resulting answers may surprise you:
In 1972, 39% of adult Catholics responded that premarital sex was “always wrong.” Among Catholics attending Mass at least once a week, 54% responded as such that year. In the most recent survey, conducted in 2008, only 14% of Catholics responded that premarital sex is “always wrong.” Among Catholics attending Mass at least once a week, 30% responded as such.
In other words, more than 70% of Church-going Catholics do not hold or agree to the teaching the Church and Scripture that premarital sex (fornication) is always wrong. Among Catholics who do not go to Mass the number is even higher at 86%.
And for Protestants?
Check this out:
For Church-going Protestants, the number of those who agree with the Biblical teaching on premarital sex is between 53 and 62%. For non-Church going Protestants the numbers are between 30 and 40%.
Read the rest, with some charts and analysis, here.