To my ISFJ (without getting graphic at all!), sex is special (his words). Very frequent at first, now only frequent when he does not have a lot of work on his plate. Very interested in finding out what I liked/my needs at first, now not as interested so much in changing things or learning anything more about that (takes comments on what I like now as a bit of embarrassing rejection, but in the beginning wanted to know all about what I liked; in other words, once he learned what I liked and does it, sees any more commentary as criticism -- it's the only touchy area we deal with as a couple). Actually pretty spontaneous as far as when we have it. Likes me to let him initiate mostly but sometimes likes me to take charge. But, if I'm too aggressive with initiating (too often when he's tired, etc.) this frustrates him.
All in all, if I respect his boundaries, it's really great.
Also, my ISFJ told me that women who threw themselves at him in the past sexually didn't garner any respect from him. He likes to be the initial aggressor.
In Socionics terms, here is how the two subtypes of ISFJs handle sex:
Sensory subtype (The Traditionalist):
(Sexual behaviour) Emotional yet restrained in expressing feelings. Capable of taking initiative if their partner finds it difficult, but only if partner will confidently reciprocate. Have many concealed complexes, which they find difficult to free themselves from; because of these they may seem cold in regards to their erotic feelings. Require both romantic and practical proofs of love, and an emotionally confident partner. Need a flexible, dynamic person, capable of removing suspicion and influencing the favorable outcome of events.
Ethical Subtype (The Moralist):
(Sexual behaviour) Disposed to constancy, honesty and stability. In relations they are tactful and attempt to fulfill the desires of their partner. Verbally express their views and concerns. Attentive to detail, are careful and distrustful. Need a partner that’s not overly demanding; partner should be frugal and aid in the creation of comfort, order and welfare. Partner should be responsive to this type's pleasures but more so to their attachments; otherwise they may possibly break relations, ignoring any agreements.