How do we know what a person's sexual orientation is?
Appearance, mannerisms and speech will often give a reasonable indication or at least raise a question mark, obviously not accurate all the time but sometimes you just know straight away. These days people can be quite open about it and I’ve had male crew members talking about their boyfriends whilst inflight.
I knew a very competent Captain who was unable to get in with two airlines in the Middle East, simply because he came across as a bit effeminate. He was married with children but first impressions did not indicate straight male behaviour, and that sort of thing does not go down well over there. Saudi Arabia has the death penalty for it.
Certain airlines are gay friendly and deal well with issues such as civil partnerships, staff travel and other matters pertaining to same sex couples. They will not tolerate any form of harassment and are openly supportive.