Essentially, Forbes brought an aspect of his professional life - students attending the University at which he works - into his personal life. This isn't wrong in itself, but he did so in the context of a magazine that long has been accused of objectifying women and promulgating indecency.
Like all faculty members, Forbes can enjoy the same freedoms as other citizens without fear or threat of censorship or discipline on the part of the University. But the consequences of his action are too significant to ignore.
In agreeing to host the shoot, he discounted the possibility that some of his current and future students might feel uncomfortable with him leading their class.
Forbes is still a professor at UNC. He will continue to teach students who know or easily could come to know that he allowed nude female students to be photographed at his home.
Now that his actions have been well publicized, it is likely that some of his students will question his character. The comfort level of his female students might waver.
To be fair, Forbes was absent during Playboy's nude photography sessions. He also expressed regret about the way in which The Chapel Hill News reported his involvement, and he apologized to those in the University community who might have been offended by his actions.
He seems to have realized his mistake, but it's too late. The damage is done.
Forbes should have considered the impact of his decision on the University and, more importantly, his students. By not doing so, he has undermined his role as a UNC professor.