Sherri Shepherd's Ex -- Rules for Our Marriage ... 1) DON'T GET FAT

Sherri Shepherd's estranged husband wanted her to sign a document promising that she would enjoy sex and not get fat ... TMZ has learned.

Sherri and Lamar Sally signed a prenup before they got hitched in 2011. We're told at some point after the wedding, Lamar wanted an amendment to the prenup, which included the following:

-- "I respect my husband's opinions and recognize him as the leader of our home. I will always speak well of my husband."

-- "I enjoy having sex with my husband. I crave intimacy with him and want to be uninhibited and free in our lovemaking."

-- "MY BODY IS IMPORTANT TO ME. I STRIVE TO MAINTAIN A HEALTHY BODY WEIGHT AND EXERCISE AT LEAST THREE TIMES A WEEK." (HIS emphasis)

-- "It is my joy to submit to my husband as a way to honor God."

Of course Sherri rejected the amendment.

Lamar filed for separation earlier this month ... Sherri has since filed for divorce. They're fighting over custody of their unborn baby via surrogate.

We contacted Lamar's lawyer ... so far no word back.