The missives come from a team of researchers from Lausanne and Zurich, who are undertaking a massive study to work out how sex has changed over the past 20 years.
They have contacted 40,000 people in their 20s, with the aim of learning about their sexual attitudes and experiences in order to analyze their impact on other areas of health and emotional well-being.
"Sexuality can not be isolated from other areas of life," said professor Joan-Carles Suris, one of the researchers behind the project which is receiving funding from the Swiss National Science Foundation (SNSF). "If someone's sexual life isn't up to their expectations - either in quantity or quality - or if there are related issues such as abuse or dysfunction, then the general well-being of that person will suffer."
Suris said the project aimed to "set the record straight and understand if and how things have changed" since the last research project on youth sexuality was carried out in 1995.
The researchers will look at how cultural changes such as the internet, medical advances such as the morning-after pill, and growing acceptance of LGBT relationships have affected individuals and their sex lives.
One in six people aged between 24 and 28 will receive the questionnaires, an age range chosen because 25 is an "age when someone can already have a certain distance on their first intimate relationships", the researchers said. Key themes in the survey will include unwanted relationships, unprotected sex, substance use, and sexual dysfunction.
Those who choose to take part can answer anonymously online by contributing to a "calendar of life", which will use visual and interactive tools to stimulate the memory and help respondents recall events.
The first report from the data will be published in early 2018 and will be followed by scientific publications looking in depth at the project's different aspects, researchers said, "where the team may well touch many taboos".