@DBActive
Who gets to decide "when biology matters?" Does it matter to a girl who is placed with biological men in a locker room? Does it matter to a school girl who is raped in the girls bathroom by a "gender fluid" male student? does it matter to women prisoners who are housed with biological men and raped by them? Or is it only the persons who want to change their appearance whose right not to be offended matters? For that matter, what committment does have person have to make to a "gender identity" to be considered a trans woman or man? Is just putting on a dress sufficient?
For some of those, I'll stick with 'I'm not sure'. The claims of rape in particular are rather exaggerated. Yes, it does happen (see awnlee jawking's post below), but straight non-trans people are far more likely to rape someone (per capita) than trans people. Rape is a problem no matter who does it, and many of the proposed anti-trans laws (e.g. the 'gender assigned at birth' bathroom legislation) make rape more likely, not less.
Remember that the alternative here is to allow someone dressed, presenting, and acting like a stereotypical male free access to women's restrooms as long as they claim to have been assigned female at birth. That, or strip searches before one can use the bathroom at all.
As to sports - nobody is saying they can't participate - they have the full freedom to partipate on teams in line with their sex. How is that any more restrictive than to them than the situation where girls and women are forced to compete against males?
Neither of those are likely to be viable options. The first is equivalent to those who said, years ago, that there was no discrimination against gays getting married. Gay men were just as entitled to marry women as straight men were, and lesbians were just as entitled to marry men as straight women were. Thus, there was no discrimination. Yeah, right.
Yet, in terms of competition, of course there are issues with parity based on physical attributes. The notion that a biological male, no matter how genuine their transition is, should be competing with biological females in high-level competition creates enormous problems.
Those can't be the only options. What high school football team is going to accept someone who's biologically male but who identifies with and dresses as stereotypically female? For that matter, what high school basketball team (of either sex) is going to accept someone who's biologically their sex but isn't the same gender? If you want to talk about locker room assaults, the number of trans people who are assaulted because they're trans is enormously higher than the number of non-trans people who are assaulted by non-trans people.
The answer is likely going to be to allow trans people to participate in sports at a participatory level, without including them in the top-level competitive categories. That's not ideal, but it's better than either of the other two alternatives. Denying people the right to participate at all is highly unfair. Giving them the right to participate, but only in a way that's completely ridiculous, is just insulting them by offering them an 'option' that isn't in any way a real option. And giving them the right to participate, but in a way that disadvantages large numbers of people, is also unfair.