In Canada it's illegal to discriminate against someone on grounds of sexual orientation or gender. I replied to an ad on craigslist I found and they responded with a boilerplate questionnaire with questions, such as "do you smoke" or "do you have food allergies?"
One of the questions was "Are you straight or LGBT?". I'm fairly certain that it would be illegal for a landlord to base the decision to rent to someone or not, based on their sexual preference. But how on Earth would it be enforceable? I don't think simply asking the question is illegal. Even if a plaintiff could prove that they weren't selected to rent the house, what can be done about it? For the land lord to rent to them because they asked if they were straight?
Also the questionnaire asked about tattoos. Is it a form of discrimination if a person is judged based on tattoos? I know some jobs require their employees to cover tattoo at work.
EDIT: the main question is, what should/can a person do if they are asked the question and are likely in the minority group? For example if someone who is gay is asked by the landlord if they are gay, and then not given the house?