Sorry if this has already answered, but I found a similar question in the HP subreddit and went to check the Wiki page on Veelas but couldn't find an answer.
I know that the 1/8th Veela Louis Delacour was male, but that could have been because of being mostly human.
There also is this line in the Behind The Scenes section of the Veela page that doesn't have a note or reference "One of the questions on the 2nd Level W.O.M.B.A.T. asks the examinee to identify which of five given statements is false; one of these claims that full-blooded male full Veela do not exist." But this line about the statements doesn't say if that is one of the true facts or not, at least not to my knowledge, so it could be the lie.
In my opinion that leaves two possibilities for how full Veela breed:
A. There are male full Veelas but we just haven't seen any in canon yet.
B. Full Veelas are some kind of hermaphrodites, they each have male and female reproductive capabilities, and that breed with each other. This would mean that not only can human males mate with a full Veela, but also human females.
Anyway, that's just my thoughts, its probably A, but it'd be cool if it was B.