Well of course, your own male hand on your own male dick isn't gay, or straight or anything else - it's just masturbation but of course you could have straight or gay fantasies while doing it or even dream of a mixed orgy.
And I am sure most men who weren't looking at the person jacking them and were therefore unaware of the sex of that person would enjoy the sensations and could possibly cum that way.
On the other hand it does seem to me that the kind of sexual pleasure we seek out is pretty intrinsic to orientation so a guy who was 100% straight would not seek out mutual masturbation with other men. Of course, that doesn't mean there is anything wrong with enjoying it, just that those who do are not at zero on the Kinsey scale and indeed, these days, many more youngsters say they are not completely straight.
When one looks at such things as college frat rituals and some of the goings on in the military is also clear that a man can engage in homosexual acts and yet lead an otherwise straight lifestyle.
For me, the most important thing to realise is that labels such as straight bi or gay should be descriptive and are too simple to capture the full diversity of human sexuality. Do what you get pleasure from and don't worry if it doesn't fit a neat label.