I have never heard of any code of conduct or rules regarding sexual relations between Priest and Matushka.
What I have seen is that Priest and Matushka have children and their children are not given any special privilege other than their proximity to their parents and Orthodoxy tends to prepare and motivate them for monasteries. So it’s not surprising that these children become monastics.
But on the subject of Orthodox children, I have posted before some interesting observations and especially to the meaning of ‘brothers’ and ‘sisters’ as labels for actual brothers and sisters as well as FIRST COUSINS. Also in the vein of this observation is the role of God Parents. God Parents in Orthodoxy have serious, very serious responsibilities. Every child in an Orthodox community has God Parents and those God Parents are responsible for the life of their God Children both when they are children and when they are adults. In fact God Parents are more often the most influential in an Orthodox person’s life.
Back to the subject, I have never heard of restrictions on sexual relations for Priest and Matushka. But I imagine some priests have to prepare for their spiritual responsibilities to their Parish and in doing so they adopt to refrain from sexual relations at certain times so as to better focus on their priestly duties. But I really don’t know; just never heard of any hard fast rule and I doubt one exists.
I have known of Orthodox priests with 14 children born from Matushka. So I don’t see a restriction here!
Periodic abstinence --- depending on how it's done -- does not generally have a negative impact on fertility; that is, unless you're purposefully abstaining at the fertile time of the cycle. Using planned periodic abstinence to enhance fertility --- as the most observant Orthodox Jews do, per Torah law ---- is also one of the features of NFP.
Intercourse can result in pregnancy only 5-6 days of the month at most. So there's a 75-80% chance that a couple that abstains a bit based on the Liturgical Calendar is abstaining at an infertile time anyway.