For me it’s safety first and fairness second.
Allowing a man to play in a women’s game of rugby is unsafe, for the women, and the women will not realistically be able to compete, and therefore it’s unfair - I’d even say it’s cheating.
I’m exaggerating to make a point, but can you imagine Mako Vunipola deciding he’s now identifying as a woman and competing against women? It’s so laughable that I’m surprised it’s even worthy of debate.