Hey now, I'm pretty sure Veelas can be guys.
A recent scouring of the HP wikia and a googling result in uncertainty - there's nothing definite out there. My instinct is that pure Veela are female-only like their mythological sources, and it
is canon that male part-Veela do not have Veela attributes, unlike their female counterparts, suggesting that male Veela are sort of... not actually Veela. Then again, if they're female-only, that raises the question of how Veela might breed. The WOMBAT test has a true or false question for which one possible false answer is "There are no male Veela" except I can't find a damn answer to the question - but it would be reasonable to assume that if there were male Veela, the question would be moot if it was common knowledge they existed, implying that they're at the very least rare/distinct from their female counterparts.
Unless some people desperately want Veela dudes, I'd say that Veela with Veela powers are indeed female-only, if only because male ones would totally contradict my headcanon.
@McHaggis I feel like you ought to do a sense-check because some things seemed a bit confusing/contradictory. Then again, the Laser of JusticeTM might require re-calibration sobriety before firing so either of us could have goofed.
Can confirm that the Laser of Justice
TM is now fully operational and deems her good to go. I think I got thrown by too many brothers and mothers in the background section.