Adult women can and do commit statutory rape against underage boys. When this phenomenon first came to light with a female high school teacher and an underage male student, it was shocking and I think many people assumed such a thing was freakishly rare.
Subsequently, a number of such cases have been publicized suggesting this kind of occurrence is not nearly as freakishly uncommon as many believed. As with men and underage girls, the complicity of the victim does not alter the criminality of the act.
The second way women can rape men is through unwilling (anal or oral) penetration of men with fingers or objects. (I don't know if unwilling oral penetration is always treated as rape in law.)
Finally, a woman can rape a man through coercing them to penetrate the woman with an eréct pénis. Though it is not easy to 'will' an erection, it's not always impossible, either. Men, like women, can become physically aroused despite not actually wanting to have sex with someone. This may occur in response to physical manipulation or other reasons. Coercion need not be through physical force, obviously; a man can be blackmailed into having sex (as one commenter described his own victimization here).
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