Dr. Jenn Mann has advice for Elizabeth Hurley -- keep your clothes on around your grown son ... 'cause it goes beyond the pale of what she considers a healthy parent-child dynamic.

The "Couples Therapy" doc tells TMZ ... even though the actress' 21-year-old son Damian is a consenting adult, the fact that he's directing his mom's sex scenes is a major boundary breach between parent and child -- at least from her POV, anyway.

With two best-selling parenting books under her belt -- "SuperBaby: 12 Ways to Give Your Child a Head Start In the First 3 Years" and "The A to Z Guide to Raising Happy, Confident Kids" -- the renowned psychotherapist is speaking from experience when she advises against simulating sex in front of your kid. In her eyes, it's just not the best way to operate.

Jenn does acknowledge that this is a Hollywood thing -- calling out the biz for its lack of boundaries ... noting that Damian directing his mom's sex scene for her upcoming erotic thriller "Strictly Confidential" really blurs the lines ... presumably between voyeurism and art.

She also tells us she finds it downright bizarre that Hurley felt liberated by her son witnessing her being intimate on-camera ... adding the only reason she'd understand EH's empowerment is if she was merely boosting his directing career. Aside from that ... she doesn't get it.

Jenn's not shy in pointing out that Liz and Damian's relationship seems to push boundaries that might be crossing into taboo territory, violating cultural norms ... and she gets why others, like herself, would have a negative reaction to the whole situation.

JM says she thinks Liz and Damian might be best suited for therapy -- and lucky for them, she's a wiz in that field.

As we reported ... EH told Access Hollywood her son Damian made her comfortable during an intimate scene where she wears some seriously skimpy clothing while getting steamy with another woman.

While most others would recoil at the idea of seeing their parents in such a scene ... Liz says she enjoyed it so much, she'd be totally down for a little more mother-son collaboration in the future.