The only problem here I think was because they don't know, or never defined, their own boundaries.
He technically didn't engage in anything sexual with Yukino—and, as he questioned before this arc and concluded in the end, he understood that S&M wasn't a "sex thing". It's a form of communication between the S and the M.
To some (like Yuka), it's about letting inhibitions free. To others (like Yukino), it's a form of self-expression. S&M wasn't about the S selfishly making the M do things, it's about the two of them communicating through these underlying desires, which does get rather physical and intimate, which was why it sometimes involves sex.
The issue was that Mahiro and Yuka never communicated, not explicitly, not verbally, about what defined their boundaries. You could argue that he cheated on her. You could also argue otherwise. It's something they need to figure out going on because I found that proper S&M really makes you think about these things, lol.
The one thing I do agree though was that it's always a good thing to find a mentor in S&M. As much as it's about exploration, the entire thing was not uncharted territory. There would be risks and ideas, people with more experience who could help you find places to start making your own journey.
That's basically Yukino's role in their journey. I can appreciate that at least.