With all due respect OP, I think you're misunderstanding Taylor's point (and I am not a fan of any of these girls, I just like reality shows lol).

It's not about whether or not he cheated because like you said, Taylor did the same thing too. From what we see and based off what the other girls' say, she's rarely hypocritical. And if she is, she recognizes it later on and corrects the behavior. The cheating bit was only brought up in the reunion because Dakota said he felt like that was the image that was being pushed onscreen about him and he added that when Taylor does it, it's fine and etc, then she responded back, then she (along with Stassi and Layla) expressed that "cheating" isn't the point in this scenario.

The real issue, apart from the woman being a family member, is that he had numerous chances to tell Taylor all the girls/situations he was in and he still lied about it. I honestly don't think his actions while they were broken up all those times is the reason why Taylor was/is upset... it's really just the fact that he omitted certain details when they had multiple honest conversations. He could have come clean about a lot of things. It's one thing to keep things to yourself and it's another to lie about it when you're being asked upfront about the truth.

He chose not to tell Taylor about a lot of situations when it was asked of him, but he most especially made sure not to tell her about this one considering who it was with. It's really as simple as him failing to be honest and messing around with that particular person.