This one time on social media I found this guy that had a thing for femboys and said he was straight. It kinda confuses me because I always thought that your sexuality is judged by what sex attracts you :/
If you're a man and like feminine guys then... no, since they're still guys. If by 'femboy' you mean a trans guy, then the answer is still no, since they are men.
He could be Gay/Bi/questioning but in denial and those 'femboys' can be a way for him to escape to have a safe valve for his attraction while convincing himself being 'straight', especially if he is still a teen-ager. Can you try to explain how those 'femboys' look like?
From what I could gather, he seems to be interested in skinny guys with long hair that crossdress in revealing clothes, sometimes they have eyeshadow. He seems to like girls too but the f-boys are more common
Some people are actually attracted to people, not genitalia. Anyway, people can label themselves however they want to - they're just labels, after all. If a guy is mostly attracted to females, but attracted to feminine males sometimes, I don't see an issue with him labeling himself as straight. Sometimes, gay men find themselves attracted to women and even sleep with them, but are still labeled as gay. I don't get the rigidity when it comes to labeling heterosexual men and their attraction/experiences. Who really cares.
Boys are boys no matter what they look like. It would be like saying girls attracted to really butch lesbian are straight because some butch lesbians look like guys.
Being pansexual myself, I'm aware of that. But, that doesn't change the fact that the standard definition of "being straight" means they are attracted solely to women. By that definition (note I said "standard definition", for as you have pointed out not everyone has the same definitions on such things), if a man likes dick, they are not straight. They may be bisexual, pansexual, asexual, or any other of many different labels... but by standard definition they would not be straight. I personally feel that most people land somewhere on the "bisexual" scale. Maybe to the far right on the scale, maybe to the far left on the scale.... but somewhere on that scale. Not everybody, but most. As you said, though, people can define themselves as they like... and label themselves as they like, it doesn't really matter as long as they are in touch with their personal preferences enough to know what they want and like and not jerk others around.
When I was young and still gorgeous I could dress in an ambiguous way and be mistaken as female. Other than long hair, longer manicured nails and little body hair I did not outwardly or flamboyantly project my femininity but straight guys still picked up on it and would come on to me. Most would reject me immediately once they realized I was male but some guys would continue to pursue me sexually even though they were adamant about their own heterosexuality. I have agreed to have sex with some of the guys I met (For personal research purposes that had nothing to do with me lusting for a hotty straight guy of course!) and in every case these guys acted and performed like, well, straight guys. They had no interest in my penis, they had zero interest in being the receptive partner and were repulsed by even the thought of me having an orgasm or ejaculating while I was with them. If I did, they did not want to know anything about it. After sex actions were off putting at first and accurately predictable later. These guys would always beat a hasty retreat from the scene, there was no after sex intimacy and sometimes the guy would become angry at himself or me. "I don't know why I did that." and "This is your fault, you made me do this to you." are things said to me. There has never been any follow up by any of the guys I had been with even when I have seen them out in public they to a guy will ignore me even being there. I have since learned these types of guys are known as MSM's or men who have sex with men but the one's who chose me did so specifically for my feminine qualities. If you were to suggest or tell any of these guys they were gay or bi I am pretty sure they would beat you to a pulp.
