Do you think it's safe to not use condoms on homosexual relationships if it's monogamous and both partners are sts free?

Yes: Stds aside, good hygiene and common sense would be my only recommendations regardless of sexual orientation.

Other health risks: Anal sex has its own collections of issues beyond STD's. The digestive tract is full of yeast and bacteria that pass readily into a penis & can trigger urinary tract infections in those with the tendency. Condoms provide some protection from these.