Some marriage mates might discreetly hold hands, but if they embraced each other during public prayer, those who got a glimpse of such conduct might be stumbled. They might think or get the impression that the couple was focusing on their romantic relationship instead of reverence for Jehovah.
Simce marriage is viewed as a threefold cord, holding hands / embracing should symbolize a couple's coming together with Jehovah as a threesome. Nothing wrong with that =)