I've got an ISTP wife. It's weird to be the "feeling guy" all the time (even though I am a week F). She looks at me, when I am trying to discuss some topic of importance, and tells me, how emotional I am. I feel much sympathy for the women who are usually the ones in that position. I have never been in a relationship where I am in this role -- like I said, it feels weird.
Because she is so stoic, I used to get jealous a lot, in the early stages of the relationship. She would go off on ski trips and I'd wind up saying some stupid jealous thing to her, because (a) i wished I was a skilled enough skier to go on that trip, and (b) she was so darn stoic. I was never 100% sure how she felt, because she showed emotion so rarely.
These days I am more philosophical about it. She seems to love me as much as she is capable of loving anyone, even though she has no desire to talk about those feelings. Honestly, that's still a bitter pill for me, but hey, that's how life is sometimes. Instead of words, she gave me facts. Years of consistent dedication, and she almost never complains about personal discomfort or hardship. I just learned to believe in her, and love her as she is.
You should also know that she suffers a bit from depression. That exacerbates the low levels of emotional display.
Interestingly enough, my INFJ sister also married an ISTP. He seems to have lost interest in interacting with people in general, preferring the company of his TV. She has learned to deal with it by having her own life.