The Pew Forum on Religion breaks out religious affiliation by marital status. According to their detail table, the highest percentage of never marrieds are amongst atheists (37%), almost twice the rate as in the total population (19%). This might be at least in part due to the younger age of atheists (37% are between 18 and 29 years old compared to 20% in the total population). 30% of secular unaffiliated/atheist/agnostic are never marrieds; 45% are currently married. 29% of this group is above 50 years old, so not all these singles are young adults.
I think the high presence of singles, especially never married people, underscores the need to discuss alternatives to marriage, as well as singlism, amongst secular humanists.