I think it can still be straight men if they masturbate together. They are still technically beating off to women (presumably) porno.
If in any point they beat each other off, then i would condone that a homosexual act.
I believe it is done a lot more often then it appears however, America in particular have this "ugh, that activity, or this activity is gayyyyyy" mentality. In other countries, there are less concerns of male to male interactions, i.e. hugging, or even bath houses where all the men are naked and sharing the same bath.