In fact, the award-winning, much-celebrated sex guide Enduring Desire by marital and sex therapists Michael Metz and Barry McCarthy points to research that found that the best sex occurs in couples who have been together for 15 years or longer! All your insecurities and vulnerabilities will come to the surface in a long-term relationship. In a long-term relationship, the pressure lifts. Does she really like me? Is he weird? When was my last wax? Which pants am I wearing?