The statements in question relate to whether different social groups have, on average, different rates of discount. The rate of discount is how an economist quantifies how a person evaluates the future. A higher discount rate means the present is more important and the future is less important (and vice-versa).
Hoppe's offense was to remark that homosexuals, on average, probably have a higher discount rate and therefore save less for the future. This actually is a rather sensible position for an individual who may have a higher degree of uncertainty regarding their exposure to HIV in the past. In fact, an economist might even argue that it isn't rational not to have a higher discount rate in such a case.
But, a student complained directly to administration after the lecture. After notification, Hoppe explained his position to his class, and noted that he was speaking in terms of averages across groups. The student then filed a formal complaint that Hoppe did not take their position seriously.
The ACLU is defending Hoppe's right to such speech. In fact, the executive director of the ACLU of Nevada is quoted as saying:
...Academic freedom means nothing if it doesn't protect the right of professors to present scholarly ideas that are relevant to their curricula, even if they are controversial and rub people the wrong way.
Note the similarities to the infamous Roberds' case still ongoing at Southern Utah University. Roberds was fired and Hoppe is in the process of being disciplined. Roberds and Hoppe both claim free speech is the issue.
But note the critical contrast: Roberds is not being supported by any public advocacy groups, while Hoppe is. Hoppe's right to say what he did is even being defended by a group that has says publicly they don't agree with him (see here). Alternatively, there are published reports that Roberds has not been forthcoming with at least one group that wants to help him. Now, Roberds is under no obligation to accept such help, or to open his case to outsiders, but this should make clear that help is available, quickly and promptly, to those who want it or deserve it.