At the risk of going off at a tangent - Homosexuality the evidence suggests is developmental rather than genetic, so when someone says they are born gay it's not strictly true.
Although there is little evidence that it's purely environmental otherwise Homosexuallty would not exist in places where it's proscribed. It can be incidental based on environment, thus the incident rates of homosexuality when people of the same sex are kept in close confinement together (like prison).
Going back to Rape, in terms of male on male and male on female the overriding element is that of violence, power and control. Some people derrive sexual satisfaction from exerting control over others. But equally it's not necessarily sexual at all - it's an act to assert dominance and make the victim feel weak and humiliated.