As the other replies have said, she's insane (ex. sucking on vials in a superscience lab). Raising the boys with Myra was Rusty's original plan. That and hot pants.
Outside of Myra's personal faults, Rusty seems to be pretty independent. Before their relationship soured, in The Invisible Hand of Fate, Rusty seems deeply annoyed by Myra's constant harassment (ie. phonecalls) of him. He accuses her of acting like his mother - and it makes sense why he, as someone raised without one, would find such behavior daunting. Rusty has spent his entire life alone. He was a kid surrounded entirely by adults who were more than happy to ditch him to go to swinger's parties. After his father died, Rusty went back home to the compound where he lived on a massive estate with no neighbors in the immediate vicinity and worked at home. College was probably the only time in Thaddeus' life (up until current events) where he actually had regular society with others. It's hard for someone used to that kind of lifestyle to adjust to someone who wants to know where he is every second of the day.Doc's honestly a bit of a walking contradiction in this regard. He's desperate for affection - see how quickly he rips his clothes off and starts posing for Evil Andy Warhol because he was complimented. Simultaneously, however, Thaddeus cannot deal with any of the actual responsibilities that come with being loved by someone - like being accountable to them for your whereabouts.
We also see that Doc has pretty high standards regarding women. His response to Dr. Quymm being an epileptic was one of disgust - and this woman is a childhood friend with a big heart, an astounding intellect, dat ass, and a super-science business and lifestyle of her own. He has a similar reaction to the equally smart and sweet Sally Impossible because she turns invisible.>awful looking
Don't speak of my husbando like that.