The guards' actions are clearly illegal:

It is a crime for a prison staff member to engage in any sexual activity with an inmate, and consent by the inmate does not matter under the law because of the imbalance of power in the relationship.

The Prison Rape Elimination Act of 2003 mandates that prison officials make prevention of sexual abuse in prisons, jails and cellblocks a top priority. The Federal Bureau of Prisons manages 115 facilities and houses 171,000 inmates.

What other sexual assault cases would result in these minimal penalties?

Since a change in the law in 2006, the percentage of cases that assistant U.S. attorneys accepted for prosecution has risen by more than 12 percent. In the cases in which criminal charges were filed, 83 out of 90 resulted in convictions. The vast majority of the penalties, however, resulted in sentences of less than one year of prison time under both the older and newer laws. (my emphasis).

And claims against women guards are rising at at a rate higher than their representative numbers in the prison work force.

What does the BOP have to say about it?

Harley G. Lappin, director of the Bureau of Prisons, pointed out in a letter that prison officials attribute the rise in allegations "to our efforts to educate and encourage reporting of these incidents" by inmates and other prison staff members.

In other words, sexually abusing inmates has always been business as usual, the BOP is now just encouraging it to be reported?

What does the Inspector General recommend?

Fine urged the Justice Department, the Bureau of Prisons and the U.S. Marshals Service to step up their outreach to prosecutors and to reconsider policies in some prison facilities that isolate inmates or transfer them after they raise allegations of sexual misconduct by corrections officers and other members of the prison staff.