I’m going to try and answer this question with maturity. No, that would not make him gay. Men have a G-spot, or the Gräfenberg Spot, in their prostate that, when stimulated, leads to sexual arousal. What he particularly enjoys does not necessarily make him gay unless he feels sexual attraction towards men instead of women. However in this case, if he feels sexual attraction towards women AND men, he is bisexual.
I don't think he would be considered gay. Just like not every woman can come from penetration. Since the prostate can be touched thru ànal penetration he gets a bigger stimulus being pegged with a strap on. It's more a pleasure thing rather than orientation. And if anything,since he's enjoying se.x with a female partner he would be bi-curious but for sure not gay. Hope it makes sense what I said,i'm too tired.
But why bi-curious? He’s never had a sexual attraction to a man. Also he can cûm from “regular” woman on man sex, he just prefers it when they use a dildo.
Because maybe he doesn't actually realize but if he would be put in the proper situation he might not reject the idea of sleeping with a man?