Katnis can you really not see it? It's girls and wine who lose, either way.
Transitioning transboy (biological female) competes against other biological females, but with the advantage of being on testosterone: girls are disadvantaged.
If the transman competes against boys, are the boys disadvantaged? Probably not, because the transboys's testosterone levels are only trying to approximate the levels the boys have anyway, and the transboy still has a female body which is unlikely to beat a male body in a test of brute physical strength.
Transitioning transgirl (biological male) competes against biological females, with with the advantage of naturally occurring testosterone levels: girls are disadvantaged. Even if T levels are reduced by 'transition', the girls with their female bodies are unlikely to beat a person with a male body in a test of brute physical strength.
If the transgirl competes against boys, are the boys disadvantaged? Probably not, because if there's been no hormonal 'transition' then they are biological (i.e.with the hormones and physiology typical of males), and if the transgirl has had hormone treatment, if anything the boys who haven't will have an advantage.
So either girls lose out, or girls lose out (and so do women in the adult context). So some people are saying "wait, let's figure out a way around this where girls aren't unfairly disadvantaged," but that position is dismissed as transphobic. Why is it so wrong to care about the girls?