Yeah I meant gender when I said sex.
Ah ok. That makes much more sense - it's a bit early for me!
WRT 'believing a person can be trans', it depends what you mean by trans, as the word has a very wide definition now.
There are people who suffer from gender dysphoria, sometimes to the extent that those people want to make radical changes to their bodies. Some may choose to take cross-sex hormones, some may choose to get surgery on their breasts/chest, a few choose to get genital surgery.
These treatments aren't risk-free, and aren't always successful in stopping the dysphoria. There may be other mental health issues at hand. But, for the most part, we accept that adults have the right to do whatever they want to their own bodies (apart from conditions such as anorexia, or that tattooist/body modifier who went to court for splitting someone's tongue even though they wanted it done).
One of the hot topics here is the medicalisation and subsequent sterilisation of children, sometimes pre-puberty, who appear to be suffering from gender dysphoria (I say appear to be because I do think there are other issues at play in at least some of these children.
But aside from those suffering from gender dysphoria, trans also includes people who only 'identify' as the other sex and make no changes to their appearance. It includes men who identify as male one day and female another. And others.
I have to go now but I'll be back later!