Sexual desire discrepancy (SDD) occurs when partners have different levels of sexual desire and are distressed by it. It is one of the most common reasons couples seek help, but no proven dyadic treatment currently exists. This study will test STEP, an online program designed to support couples with SDD. The investigators will compare two formats of STEP: one with therapist support and one self-guided, against a waitlist control group. The investigators will also assess how satisfied couples are and how they engage with the two online formats. This study will generate new data on how effective STEP is with and without guidance, and inform treatment guidelines.

This study aims to evaluate the effectiveness of an 8-week online intervention (STEP) designed to reduce distress related to SDD in couples. Two formats of the intervention will be tested: a therapist-guided version (STEP-T) and a self-guided version (STEP-NT). Couples will be randomly assigned to one of these formats or to a waitlist control group.

The study will assess whether both versions of STEP reduce SDD and associated distress, and whether improvements are maintained over a 6-month follow-up. The investigators will also examine participants' satisfaction with the program, engagement, and retention rates.

In addition, the investigators will explore the psychological factors (e.g., quality of sexual communication, emotional intimacy, interoceptive awareness, and self-compassion) that may explain how the intervention works, as well as whether its effectiveness varies based on relationship duration and participants' perceptions of the treatment's credibility.

Finally, the study will gather exploratory information about potential contributing factors to SDD and how couples experience desire-related challenges, using structured worksheets.

The investigators hypothesize that: a) Both STEP-T (therapist-guided) and STEP-NT (self-guided) will significantly reduce sexual desire discrepancy (SDD) and related distress, compared to a waitlist control group; b) Participants in both intervention groups will show improvements in sexual satisfaction, relationship satisfaction, and sexual pleasure; c) These improvements will be maintained at 6-month follow-up; d) STEP-T will produce greater improvements than STEP-NT; e) Participants will report moderate to high satisfaction, consistent homework engagement, and retention rates above 70%, with higher engagement expected in the STEP-T group; f) Increases in interoceptive awareness, emotional intimacy, and sexual communication will predict improvements in SDD and distress. Improvements in self-compassion will predict reduced distress; g) Longer relationship duration and higher perceived treatment credibility will be associated with better outcomes.

Once both members of the couple have consented to participate in the study, they will be emailed a Qualtrics link to individually complete a baseline battery of questionnaires (Time 1) and their alphanumerical code to both keep their answers confidential and make researchers able to associate their answers for statistical purposes. In the baseline battery of questionnaires, participants will complete demographic questions (e.g., age, marital status), measures assessing primary and secondary outcomes, measures of mediators, and of relationship duration (moderator). Once completed, participants will be randomized to STEP-T, STEP-NT, or a waitlist control condition. Participants will be informed about which treatment type they were randomized to via e-mail. Couples initially randomized to the waitlist will complete a second baseline battery of questionnaires (including the same measures of the first baseline battery) after a 10-week waiting period. No treatment will be given to waitlist participants during this waiting period. After this period, they will be randomized to either STEP-T or STEP-NT and will be informed about which treatment type they were randomized to via e-mail.

After scheduling the start date of the treatment, couples in the STEP-T group will meet online with the therapist via videoconferencing software (secure university Zoom account) for 120 minutes/week for 8 weeks. Couples in the STEP-NT group will also have their treatment start date scheduled and will receive one self-guided module per week via email over the course of 8 weeks. Treatment for both groups will begin within one week following the completion of the baseline assessment battery. Participants in both STEP-T and STEP-NT will individually complete, immediately before the start of the next module (for STEP-NT)/session(for STEP-T), an online measure and a homework checklist assessing their engagement with the previously completed module.

In the event that a session (STEP-T) or a module (STEP-NT) is rescheduled, it will preferably be made up within the same week it was originally planned. If that is not possible, the session/module will be made up the following week, in order to minimize the impact on the overall program schedule. Couples in STEP-T will complete treatment credibility measure immediately after session 1 while those in STEP-NT will complete it immediately before receiving module 2. Before Session 3, both members of the couple will be asked to email their completed 3P and Basson's sexual response model worksheets. Before Session 4, both members of the couple will be asked to email their sexual beliefs worksheets. Before Session 5, participants will be asked to email their CBT diamond worksheets and their biases worksheets. Time 1 and an additional questionnaire (measure of treatment satisfaction) will be readministered 2-4 weeks after treatment (Time 2), and again at 6-month follow-up (Time 3; except measure of treatment satisfaction).

The study will be administered entirely online. The research team will follow up with participants who do not respond to emails, fail to complete questionnaires, or miss a session without prior notice, sending up to three reminder emails. If participants do not respond after three follow-up attempts, they will be excluded from the treatment.

Inclusion Criteria:

1. Be a couple;
2. Be in a romantic relationship for at least one year;
3. Be 19 years of age or older;
4. Experience perceived and/or actual SDD with personal and/or relational distress reported by at least one partner;
5. Be fluent in English;
6. Be willing to participate in 8 weekly online treatment sessions;
7. For couples in long-distance relationships, be able to meet the partner in person and via video call to complete the couple's homework;
8. Be willing to abstain from starting any new treatment for SDD for the duration of the study;
9. Be willing to voluntarily complete questionnaires administered before, during, and after the treatment;
10. Have access to an electronic device capable of running Zoom platform (if randomized to STEP-T);
11. Have a stable internet connection.

Exclusion Criteria:

1. Cases where the discrepancy is attributable to at least one partner identifying as asexual (asexuality is not considered a sexual dysfunction; APA, 2013);
2. Presence of untreated psychiatric conditions at the time of recruitment (e.g., depression) that would interfere with participating in the treatment and homework;
3. Receive dyadic psychosexual treatment for SDD elsewhere during the study period.

There is no plan to share individual participant data (IPD) for this study, due to the sensitive nature of couple-level data and the associated risk of re-identification.