I have a sincere question concerning lust. Is it a sin to have a strong sexual desire for your husband? Adding to that, is it a sin to desire 'rape fantasies' with your husband? I used to have rape fantasies before I married, and now that I'm married, I've often had these fantasies about my husband. I don't have sexual thoughts about anyone else but him, so is this wrong? Thank you so much for your time and God bless.
"Lust" means to have a very strong desire, but we generally use it in the context of desiring something that is not lawful. Having a strong sexual desire for your husband is perfectly fine because sex is lawful within marriage. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). But "lust" isn't the best word to describe the way you feel about each other.
Fantasies of your husband coming to you with strong desires for sex isn't bad, but again, you use words that have the wrong connotations. One of the reasons rape is wrong is because the man displays no love for the woman he rapes. She becomes merely a means for him to ejaculate and there is no concern for her comfort, safety, or enjoyment. I suspect you've been reading too many romance novels that do not portray the realities of rape accurately.
But beyond all of this, the word "rape" carries the meaning the sex is not consensual and illegal. You are married to your husband and that marriage carries with it the implicit consent to have sex. "Let the husband render to his wife the affection due her, and likewise also the wife to her husband. The wife does not have authority over her own body, but the husband does. And likewise the husband does not have authority over his own body, but the wife does" (I Corinthians 7:3-4). What you are fantasizing is a violation of this command.