okay I looked it up. you are partially right:
Article 176 of the Penal Code provides that “[a] person who, through assault or intimidation, forcibly commits an indecent act upon a male or female of not less than thirteen years of age shall be punished by imprisonment with work for not less than 6 months but not more than 10 years. The same shall apply to a person who commits an indecent act upon a male or female under thirteen years of age.”3 - See more at: http://www.impowr.org/content/current-legal-framework-rape-and-sexual-assault-japan#sthash.oseLPlu2.dpuf
but there's an age limit. that's pretty messed up. And it doesn't technically fall under the law for "rape" how sad.
What I find a bit strange is how sexual harassment/assault towards men is taken so lightly in many yaoi series. I'm sorry but I just don't get it.