So you are right men who are 'pegged' by women are indeed, not 'gay'. And homosexuality does indeed take into account sexual practice. But 'gay' and 'homosexual' are not synonymous (although this is the sleight of hand that is being attempted here.) 'Gay' has existed for all of maybe 50 years whereas homosexuality has been recorded for 6500 years.
Thus it derives all meaning from 'attraction' and none from sexual practice.
But that is only because of the extremely narrow definition of 'gay' used in the USA, where it was coined so they do have a right to define it. Indeed they can, and the men who receive anal sex in this way, as you say, are not 'gay'.
