Gene Simmons' oldest son is coming to his dad's defense -- sort of -- calling him a cheater, egomaniac and sometimes abrasive ... but saying he's absolutely not a sexual abuser.
Nick Simmons was in WeHo Thursday and sounded off on the sexual assault lawsuit filed against the KISS rocker last week.
TMZ broke the story ... a woman -- who identified herself as Jane Doe -- sued Gene last week claiming during an interview in November he forcibly placed her hand on his knee, touched her neck and grabbed her ass.
Nick admits his dad has a reputation as somewhat of an asshole ... but assures us, he would NEVER do something to a woman against her will.