The reasonableness requirement is written into the EA2010 for the duty to make adjustments for disabled people. I think that they limited the duty to what is reasonable because these statutory duties were placed wrt disabled people but not for other protected characteristics.
The first requirement is a requirement, where a provision, criterion or practice of A's puts a disabled person at a substantial disadvantage in relation to a relevant matter in comparison with persons who are not disabled, to take such steps as it is reasonable to have to take to avoid the disadvantage.
The second requirement is a requirement, where a physical feature puts a disabled person at a substantial disadvantage in relation to a relevant matter in comparison with persons who are not disabled, to take such steps as it is reasonable to have to take to avoid the disadvantage.
The third requirement is a requirement, where a disabled person would, but for the provision of an auxiliary aid, be put at a substantial disadvantage in relation to a relevant matter in comparison with persons who are not disabled, to take such steps as it is reasonable to have to take to provide the auxiliary aid.
I haven't had a chance to look at the guidance carefully, and I'm having problems with my eyes at the moment so take this with a pinch of salt but I think the following is relevant. Based on the following, I think there is a reasonableness requirement for transgender discrimination.
Justification for separate and single-sex services
[. . . .]
13.126 However, it may be that offering alternative arrangements is not reasonably possible for the service provider (including a person providing a service in the exercise of public functions) or that doing so would undermine the service that is being provided. This may be because of the type of service being provided, the needs of the service users, the physical constraints of any building, or because of the disproportionate financial costs associated with making those arrangements.
Example
13.127 In the example in paragraph 13.125, the women who have requested women-only classes also ask for single-sex changing rooms. The gym is in a small, shared studio space which provides mixed changing facilities with private cubicles for changing. The cubicles have floor to ceiling lockable doors and there have been no complaints about inappropriate conduct in the changing rooms. The service provider determines that providing single-sex changing rooms is impractical because of space constraints and the disproportionate cost. Since the existing changing rooms enable users to change in privacy, the current arrangement is likely to be proportionate and lawful.
Example
13.128 A women’s centre provides a gym predominantly used by Jewish women who have religious objections to sharing a gym with men. The gym considers whether to open the gym to men on certain days, or to open the gym on a mixed-sex basis on certain days.
However, the centre decides to offer the gym only to women because the overwhelming demand for the service is from Jewish women and there are numerous other gyms in the area that cater to men and trans women. This service would exclude men and trans women, but this is likely to be proportionate and lawful.
The gym should also consider whether a service user may have a reasonable objection to a trans man accessing the gym and, if so, whether it is necessary to exclude them. This will be a question of proportionality and will depend on all of the circumstances, including the extent to which the trans person presents as the opposite sex. For this reason, they should only consider doing this on a case-by-case basis.
Read paragraphs 13.145 to 13.147 for further detail.
[. . . .]
13.141 It may be that offering alternative arrangements is not reasonably possible for the service provider (or person providing a service in the exercise of public functions) or that doing so would undermine the service that is being provided. This may be because of the type of service being provided, the needs of the service users, the physical constraints of any building, or because of the disproportionate financial costs associated with making those arrangements. The service provider may take account of the fact that if it admits the individual, it may cease to be a separate or single-sex service (read paragraph 13.130).
[. . . .]
13.145 If it is justified to provide a separate or single-sex service, then it will not be unlawful discrimination because of gender reassignment to prevent, limit or modify trans people’s access to the service for their own sex, as long as doing so is a proportionate means of achieving a legitimate aim (schedule 3, paragraph 28).
13.146 For example, a trans man might be excluded from the women-only service if the service provider decides that, because he presents as a man, other service users could reasonably object to his presence, and excluding him is a proportionate means of achieving a legitimate aim.
13.147 A legitimate aim for excluding a trans person from a separate or single-sex service for their own sex might be to prevent discomfort or distress for other service users. Service providers should consider whether other service users could reasonably object because they are worried about sharing a single or separate-sex service with someone who appears to be of the opposite sex. That will depend on all the circumstances, including the nature of the service in question and the extent to which the trans person presents as the opposite sex. For this reason, a service provider (including a person providing a service in the exercise of public functions) should only consider doing this on a case-by-case basis.
13.148 The service provider should consider whether there is a suitable alternative service for the trans person to use. In the case of services which are necessary for everybody, such as toilets, it is very unlikely to be proportionate to put a trans person in a position where there is no service that they are allowed to use. . . .
Example
13.151 A trans man attends a gym frequently and uses the women’s changing room, consistent with his sex. If the gym owner decides that he can no longer use the women’s changing room and there is no other changing room he can use, this may be a disproportionate decision. If it is disproportionate, the gym owner will not be able to rely on the exception for gender reassignment discrimination (schedule 3, paragraph 28). The trans man will be able to bring a complaint of direct gender reassignment discrimination, because he has been treated less favourably than a woman who does not have the protected characteristic of gender reassignment.
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This seems to relate to, e.g., a service provider telling a trans man that he can't use the womens and he can't use the gents because allowing the trans man to use the gents would mean that it was no longer single sex (and the provider wants to provide single sex toilets). Here, I think the guidance is suggesting that barring someone from both without providing an alternative will almost always be disproportionate.
I think that makes sense because a service provider could usually have alternative arrangements without expensive construction or modifications. For example, they could offer to escort the trans man when she uses the ladies.
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Communal accommodation
13.158 When excluding a person from use of communal accommodation because of sex or gender reassignment, the service provider, person exercising public functions or association must consider:
- whether and how far it is reasonable to expect that the accommodation should be altered or extended or that further accommodation should be provided, and
- the relative frequency of demand or need for the accommodation by persons of each sex (schedule 23, paragraph 3(3))
13.159 Excluding a person from use of communal accommodation provided for their own sex because of gender reassignment will only be lawful if it is a proportionate means of achieving a legitimate aim (schedule 23, paragraph 3(4)). The matters which a service provider, person exercising public functions or association should consider are similar to those set out in paragraphs 13.113 to 13.152.