deep prayer could be understood just as a layman’s way of saying “very intimate prayer”
i would think, when done with a person of the opposite sex, you open up your soul deeper and might say things during prayer you would never say otherwise. to open your soul to someone in such a way that doesnt need to be done might be inappropriate, however, i know 2 individuals who have done this, one catholic one protestant, and it has probably caused very bad problems for them. not being together for too long, convinced they want to marry eachother, being very attached to the other, perhaps are very “needy” people.
very holy indeed, but spiritually intimate with eachother i suspect. only for marriage? i dont know. but i think that one’s prayer should not be abused and used in order to draw you closer to someone, but only to draw you closer to God (in the stages before marriage) and the closer you come to God, the closer you both come to eachother.
just my thoughts.
then again, praying deeply is being honest. might be easy to get carried away though.
pray for discernment on what to pray for and how to pray