Let's take some examples into consideration.

1. Autofellatio. That's sucking your own dick. Yes, it can be done and you can likely do it too with determination and flexibility training. Ask just about anyone and you will be told it's gay as fuck, you're literally sucking dick. They'd be correct. You probably know it's gay as fuck too, no need to ask others.

2. "I'm thinking about a woman so it's not gay". You may be thinking about a woman when you're fucking a goat too, that doesn't mean you're not fucking a goat. Ultimately a man is jerking you during masturbation and you're stroking a cock.. that happens to be yours. Notice how that's gay as fuck.

3. "It's not someone else's cock". Ok then, grab a realistic dildo and stroke it rigorously and sensually or whatever you do when you masturbate. Yes, stroking a dick. Notice how that's gay as fuck.

4. When masturbating, you get 10% the sensation of penetrating a real vagina and 100% the sensation of stroking a real penis.

If you think masturbation is not gay, you have an obligation to suck your own dick too in order to be consistent.