(RNS) Unchurched Americans have high expectations that they will have sex on Valentine’s Day. Lutherans, Presbyterians and other mainline Protestants? Not so much.

A new study from the Washington-based Public Religion Research Institute, conducted in partnership with Religion News Service, shows that 57 percent of religiously unaffiliated Americans think sex is in store for them on the holiday of love.

That compares to 51 percent of Catholics who predict Valentine’s Day sex, 48 percent of white evangelicals and 40 percent of white mainline Protestants.

What’s going on -- or not going on -- between the sheets for white mainline Protestants?

“One thing you have to remember about mainline Protestants is that they tend to be older and be in longstanding relationships, and both those things are negatively correlated with having sex on Valentine’s Day,” said Daniel Cox, PRRI’s research director.

The study’s authors also asked people of various denominations what goes into a successful relationship.

Most Americans -- with the exception of white evangelical Protestants -- don’t see a couple's differing religious beliefs as a significant stumbling block for a relationship or marriage. The bigger problem, they say, is an unsatisfying sex life.

Of those surveyed, 54 percent said an unsatisfying sex life is a major problem for a relationship or marriage, while only 29 percent cited a couple’s differing religious beliefs as a major problem.

But only white evangelicals -- 56 percent -- seemed to home in on religious difference as a big relationship issue. That doesn’t mean they’re not concerned about a bad sex life -- 57 percent see it as a major problem.

Catholics also stand out in the study: Relatively few Catholics -- 19 percent -- consider differing religious beliefs a big concern for a couple. The Catholic Church, meanwhile, encourages a shared faith and typically frowns on a Catholic marrying a non-Catholic within the church.

But Cox said this finding is not surprising in light of American Catholics’ tendency on some issues -- such as gay marriage -- "to differ with official positions of the church while also affirming their identity as good Catholics."

The survey, of 1,021 Americans, was conducted between Feb. 6-10 and has a margin of error of plus or minus 3.1 percentage points.