Introduction Despite the legal recognition of gay men's right to marry and form families in numerous countries, they remain at risk of mental health impacts from prejudice. This novel experimental study aimed to harness social influence theory to improve heterosexuals' attitudes towards gay parents. Method Three hundred and forty-seven community members (M-age = 38.7 years, 52% male) and 249 university students (M-age = 19.9 years, 42% male) volunteered for an online quasi-experiment in mid-2018. Participants were randomly allocated to reading a vignette that contained either a highly supportive group attitude about gay parents, a less supportive group attitude about gay parents or an unrelated filler-task. Results The study found that (i) exposure to a high-support condition improved individuals' attitudes towards the social context of gay parents, compared to baseline, within the student sample; (ii) males reported significantly less positive attitudes towards gay parents than females in the community sample; and (iii) religiosity significantly moderated attitudes towards gay parents' parenting skills among those in the baseline condition and not among those the high-support condition, in both samples. Conclusion The findings suggest that group attitudes can be harnessed to mitigate the negative relationship between higher religiosity and individual attitudes towards gay parents. The findings are important in the aftermath of divisive debates about legalising same-sex marriage and parenting. The study findings point to the potential for religious groups to share overtly supportive group messages about gay parents, rather than staying silent, in order to prevent the harms of prejudice towards this vulnerable population.