However my husband will initiate sexual behavior that we both know the Church says is wrong: mutual masturbation. I do not initiate behavior like this when I am fertile or otherwise. My husband does and he knows it is wrong. Am I supposed to remind him every time this is a sin? Am I supposed to refuse him? I have conflicting thoughts about the verses in scripture saying our bodies belong to our spouses now.
You are supposed to refuse and resist cooperation with sin, which in this case happens to be your husband’s initiative to sin. Masturbation, whether solitary and mutual, whether unmarried or married, is gravely disordered and a denigration of the purpose and meaning of our human sexuality and a grave affront to the dignity of our person’s made in the image and likeness of Christ.
Here are a couple of scripture verses for your husband to consider regarding the gravity of leading another into sin and the charge given him to love his bride as Christ loved the Church:
"Whoever causes one of these little ones who believe in me to sin, it would be better for him if a great millstone were hung round his neck and he were thrown into the sea. And if your hand causes you to sin, cut it off; it is better for you to enter life maimed than with two hands to go to hell, to the unquenchable fire. And if your foot causes you to sin, cut it off; it is better for you to enter life lame than with two feet to be thrown into hell. And if your eye causes you to sin, pluck it out; it is better for you to enter the kingdom of God with one eye than with two eyes to be thrown into hell, where their worm does not die, and the fire is not quenched.
Mark 9:42 – 48
Husbands, love your wives, as Christ loved the church and gave himself up for her, that he might sanctify her, having cleansed her by the washing of water with the word,
that he might present the church to himself in splendor, without spot or wrinkle or any such thing, that she might be holy and without blemish. Even so husbands should love their wives as their own bodies.
Ephesians 5: 25-28
Here are a couple of scripture verses for you to consider regarding the seriousness of going along with sin, and that the ends never justify the means, i.e., recourse to evil that good/hoped for good may come of it:
But fornication and all impurity or covetousness must not even be named among you, as is fitting among saints. Let there be no filthiness, nor silly talk, nor levity, which are not fitting; but instead let there be thanksgiving. Be sure of this, that no fornicator or impure man, or one who is covetous (that is, an idolater), has any inheritance in the kingdom of Christ and of God. Let no one deceive you with empty words, for it is because of these things that the wrath of God comes upon the sons of disobedience. Therefore do not associate with them, for once you were darkness, but now you are light in the Lord; walk as children of light (for the fruit of light is found in all that is good and right and true), and try to learn what is pleasing to the Lord. Take no part in the unfruitful works of darkness, but instead expose them. For it is a shame even to speak of the things that they do in secret; but when anything is exposed by the light it becomes visible, for anything that becomes visible is light.
Ephesians 5: 3-13
Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight, and sin which clings so closely, and let us run with perseverance the race that is set before us, looking to Jesus the pioneer and perfecter of our faith, who for the joy that was set before him endured the cross, despising the shame, and is seated at the right hand of the throne of God. Consider him who endured from sinners such hostility against himself, so that you may not grow weary or fainthearted. In your struggle against sin you have not yet resisted to the point of shedding your blood.
Hebrews 12: 1-4