Lots to be learned here.
First, it appears that she was a staff member, by that I mean Jr. to the coach. What that does is open the Celtics to litigation if she sues. Even if it is consensual, she can make it look creepy and state that she was coerced by his power. Bottom line is that pro-sports is PR driving and it would cost the Celts a LOT of money to settle.
Second, Ime's GF of ten years is a public figure. They've been engaged for those ten years plus have a child together. She's been very public as a supporter of his and the Celts. Again, really bad PR.
Third, and this is unclear, there are rumors that the affair came to light because another woman reported "unwanted sexual advances" including digital (text, probably, which is stone cold hard evidence). That resulted in an "investigation" which resulted in the affair coming to light. So it is probably not a singular event with a singular person by systemic.
Lastly, he has a clause in his contract that calls this behavior out as unacceptable as a coach of the Celts. Obviously, due to PR concerns. Gotta sell those tickets!
There is also some speculation that the reason for the suspension instead of termination, is that he'll get a job someplace else. It may not be a termination-specific offense in his contract so lots of legal maneuvering going on.
I once listened to the Celtics chief legal council speak at an event. He made a big deal out of the fact that, coming out of law school, he worked for several years with the Celts "for free" because working in sports was his dream job. Now he's making several million a year. Seemed inspiring until I later learned that he is a trust fund child of a very wealthy family. A lot of us didn't have that option coming out of college. BTW, I hate he Celtics and always have.