We also don't know what really happened. It was vague and both were fully dressed and neat when we saw them jostled from their nap.
Perhaps she simply stroked his "fingers" until he was satisfied (he was an adolescent virgin - it would have taken like 8 seconds). Or maybe, since it's Pon Farr, they hit each other with sticks until she pretended she was dead.
Actually having them do the serious on the Genesis planet to the point she was impregnated is an extremely ill-conceived and, IMO, borderline tasteless plot development.