Lurking - Thanks! I had left, but popped back today - but I didnt think anyone would have noticed, so thanks for the welcome. 
Legally in the UK a man can not be raped. But even though incredibly rare, I think most people would accept a man can be raped and would call it rape.
Yes other coerced and forced sexual activity can be just as traumatic and damaging as rape. And yes this is not always treated seriously enough.
But it is because rape is based on the idea that "real" sex involves a woman being penetrated by a penis - although in the UK, after much lobbying from feminists and women's organisations, the law was changed to include penetration by an object. Thus rape is "real" forced sex. Because other sexual acts are seen as foreplay and so not real sex, they get called sexual assault.
So the whole idea of what is called rape and thus deemed most serious, is a patriarchial construction.