It is also an issue of balance and moderation. The ideal of course, is to channel sexuality through the heart, and not just focus upon physical sexuality, for that is empty. That is not to say that two Spiritual mates should not enjoy the phyiscal aspect, and it is also totally appropriate in a Spiritual context to have passion. So allow and most definitley have passion but not lower self, carnal passion, but yet allow oneself to enjoy the physical aspect. Also besides uncondtional love and not treating another just like a piece of meat, it is important not to be selfish, and to always consider the others pleasure first, as well as your own.
Sexuality is inappropriate when it becomes an unloving and selfish act.
The other key point being balance and not overindulging. This is not to say celibacy for some could not be the right choice. Sexual bonding is important for a marriage and too much time should not go on without consumating the realtionship on an ongoing basis. Although there may be periods when less sexuality may occur which is fine as long as it is mutual. If sexuality creates more love then it is good.