It might be that those professions are so dominated by one sex that there is very little data. Finding a female welder, pipefitter or lineman... er, person, is like finding a unicorn.
Same-sex occupation/relationship matchups weren’t common enough to reach the top five in any occupation.
So the chart also highlights the top male-male and female-female job matchups for each occupation.
In other words, female -> female pairings are overemphasized in the chart, otherwise you would not see them at all.lSo it might very well be the case that there are a lot more heterosexual female welders than homosexual female welders, it's just that there are so many male welders that their connections dominate the top five.