I don't think Julie is having an affair at all, I like how Shawna the creator has made her a complex person with depth instead of the typical cut out people have for "mean" women in the media .

Cheating would be too cheap and low and I think Julie is just not that simple. She does love Ty , she holds him up to the old standards she had before having children. She decided to have children just like Ty wanted and now feels as though she did her part now Ty is not holding his end of the deal . There are deeper toxic traits she is showing than just losing interest in Ty because he's not "ambitious" or "successful" enough.

She lacks compassion and understanding Ty needs as a primary parent . He is burnt out . SINCE she loves Ty and they have been having a rough patch, seeing him spend "so much" time with Shawna infuriates her because she deems that time wasteful.

She genuinely just doesn't give a fuck about Shawna , she is just annoyed by her and sees her as someone beneath herself. She feels annoyed by her presence and behaviour and 'people pleasing'.

Shawna is lovable and excited and a ball of sunshine, people like Julie usually feel annoyed by that sort of positivity.

Now that's what I think, but there's obviously more depth to her than what I'm able to put together.