Would the wife be sinning by refusing intimacy with her husband if the husband accepts that and is not angered? And would she be sinning by refusing if she is pregnant, given that medicine acknowledges the decreased desire of some pregnant women?

This is not considered a rejection because the husband made the matter dependent on his wife's desire. The forbidden rejection for which a woman is punished occurs when two conditions are met: that the man requests his need and shows his displeasure by her refusal, and that the woman has no valid excuse for refusing. If the husband accepts her refusal, or if she has an excuse such as an illness that prevents her or makes it difficult for her, then there is no sin upon her.