David Geffen‘s divorce is getting nasty … because his estranged husband is suing him for breach of contract and airing a bunch of alleged dirty laundry.

Geffen’s ex, David Andrew Armstrong (AKA Donovan Michaels) beelined it to court Tuesday and filed an explosive lawsuit against the entertainment mogul … claiming the 82-year-old billionaire was “masquerading as a white knight while hiding behind wealth, philanthropy and fame.”

In the docs, obtained by TMZ, Armstrong claims they first met on the website Seekingarrangements.com … he says Geffen paid him $10,000 for sex on the first night, then began treating him as “a living social experiment.”

Armstrong claims Geffen saw him as an object to exploit … a young man whose trauma could be weaponized for Geffen’s “personal gratification and public image.”

He says his relationship with Geffen was a “sick game” where he became a “prop in Geffen’s theater of virtue, paraded around as evidence of Geffen’s supposed altruism, while privately used as a sexual commodity.”

Armstrong claims Geffen told him he loved him and that they would treat one another as life partners and share all earnings and accumulated property equally … and he says Geffen told him he would financially support him for life. Armstrong says he gave up his modeling dreams to fully dedicate himself to Geffen’s alleged promise.

TMZ broke the story … Geffen and Armstrong did not have a prenup when Geffen filed for divorce in May after less than 2 years of marriage.

In the docs, Armstrong says the lack of a prenup reinforced his belief the relationship was genuine and enduring … but he says it was nothing more than a “transaction” to Geffen.

Armstrong says he gave his body, love, labor and youth to Geffen for years but “could not take the abuse from Geffen, nor keep up with Geffen’s proclivities, and suffered from addiction.”

He claims when things got difficult, when he no longer fit the “carefully curated narrative Geffen had crafted for his social word,” Geffen cut him off, demanded a divorce and “breached his years-long promises and agreements.”

As we first reported … a fight over money is brewing here, with Armstrong filing docs strongly suggesting he wants a chunk of Geffen’s fortune, which is estimated at $8.7 BILLION.

In his suit, Armstrong goes after Geffen for unspecified compensatory and other damages.