For example, in a small Canadian undergraduate sample, 72% of men and 78% of women who had a one-night stand or sex with someone known less than 24 hours reported experiencing some level of regret about their hooking up (4).In another sample of female undergrads from a Midwestern U. university, those who had engaged in several different hookup behaviors (including intercourse in a one-night stand, intercourse with someone known less than 24 hours, and receiving oral sex from a little-known partner) reported higher overall sexual regret than those who had not engaged in each of those behaviors (2).
(For an additional undergraduate example, see here).
It is also true that hookups are regretted more than romantic sexual interactions, at least for women.
As the graph below shows, among 100 female freshmen at a Northeastern university who had experienced both romantic and casual sex (defined broadly as not in the context of dating or in a romantic relationship, and with no mutual expectation of romantic commitment), the most recent hookup was regretted more and enjoyed less than the most recent romantic encounter (3). But That Regret Is Not Very Strong The fact that hookups are often followed by some regret, and more so than romantic sex, is only half the story.
"It's just irresponsible that states have unconstitutional laws on the books." But don't expect these states to get moving anytime soon: The Supreme Court struck down the ugly ban on interracial marriage in 1967.
Some amount of post-hookup regret is indeed common.