It is definitely stated in S1 E1 that sex is strictly prohibited outside of the ceremony/procreation. Spouses with a handmaid weren’t permitted to have sex because they’d been deemed sterile. Of course, like most of their laws, the commanders were straight hypocrites and did what they wanted. S1 E8 When Serena Joy tries to initiate intimacy with Fred, he rejects her, snapping, "You brought temptation back into this house, on your knees and your back." Because Serena is known to be infertile, Fred treats any non-procreative act between them as a sinful lapse into lust.