Because masturbating is considered a very personal/private thing to do and is generally only shared with the people you're having an intimate relation with. And yes straight men touch their own dick since it's part of their own body, just like me, I'm quite comfortable with my own body. I'm physically attracted to women, if I were a girl with this state of mind I'd be gay because I'm physically attracted to women. Touching my own body, regardless if I have a penis or a vagina I'm still attracted to women. The thought of sexual activities with men is making me feel quite uncomfortable and therefore I know I'm not attracted to men.
So, if you would use a dictionary it's quite easy to conclude whether someone is gay, bi or straight by their actions. So touching your own penis, elbow, eyelid or kneecap doesn't define someone being gay or not.
In society, most men are heterosexually active thinking, fantasies are clearly of the opposite sex. Although, when men masturbate we are groping, caressing, stroking, jerking a dick in reality, like a member of the same sex, having the same sensations. If you have experience with the female clitoris, you should have discovered that resembles a tiny penis. Mentally speaking, you can think of your cock as a clit so it doesn't seem gay that you are manhandling a cock ...right. One might conclude, as you have stated, that touching cock is kosher being heterosexual because it's yours. My topic originally never said anything about touching another man's cock. It suggested social acceptability of just masturbating together, as an art form, or sport, no matter how you define your sexuality. Humans are social beings, straights have been know to swing, trade partners, engage in orgies, etc. — masturbating with a buddy, with each one's own fantasy in mind, or watching agreed upon porn, is certainly not defined as gay. It's called jerking off with a jack off buddy, or circle jerk if in a group.