I considered myself to be gay long before I sucked my first cock. In my younger years, I thought I was attracted to women, and had sex with some over the years; not many, but some. At age 33 (give or take) I came to the realization that I didn't really enjoy pussy. I still find myself gazing at well-shaped tits - the bigger the better - and would probably suck them if the opportunity arose, but I am not interested in doing anything else sexual with a woman.
Anyway, after the realization, I began to slowly come around to the realization I was gay. I sort of always knew it, but I had kept that part of me hidden even from myself. I came out to almost all my friends and acquaintances, but It was another 8 years before I sucked cock for the first time. I knew immediately that I was born to suck cock (and later, to take cocks up my ass).
Now, does sucking cock mean you're gay? Or at least bi? Not at all. For many straight men, it's really just something they're curious about, and even if they keep it up, if they're not attracted to men in general, I don't consider them to be gay. Maybe bi-ish, but gay, bi, straight, bi-ish, gay-ish, straight-ish - they're all just labels. You do what you want to do, and don't worry about the labels. If you want to suck cock, by all means give it a try. You may like it or you may not like it, but either way, you've satisfied your curiosity.