Chastity within the confines of marriage means you don’t do anything to violate your marriage either with someone other than your spouse (adultery) or with yourself (pornography). Your marriage is honourable and must be kept in a place of honour.
The focal scripture is still Hebrews 13:4-
Anything that dishonors them is impure, grieves God and ultimately hurts even the spouses themselves.
A few examples are having sex in ways that hurt or humiliate, refusing sex out of spite, making selfishly excessive demands for sex, refusing the gift of children, and pretending that the spouse is someone else.”
No reason at all justifies you cheating on your spouse.
As she suddenly gone fat? Take her to the gym or become her personal trainer. Chastity is you staying true to your marital vows even if her Oby/Gyn has asked her to stay off sex for the remaining part of the pregnancy she’s carrying. Chastity is you staying put with your husband during the time of financial crisis without selling your body for money.
An adulterer is a traitor. God’s judgement awaits traitors (2Tim. 3:4). Stop defrauding your spouse. Stop breaking the vows you made to your spouse before God. Go and amend your ways if you’re guilty as charged. You can stay true and faithful in this world full of betrayal and lust.
You can stay pure if you decide to.