From the 2000s onwards, gay and lesbian characters began to be a more prominent part of TV shows, and that's as it should be.
Sexuality is a wonderfully copmlex thing and not everyone is or needs to be shown as 100% one way or the other. However, it's become common for a character who is otherwise straight to fall hard for someone of the same sex because that is their one true love.
There are people who fall in love based on personality and not gender, but not everybody, and when it's overused this turns into a stereotype of its own.
Torchwood probably started this trend back in 2005 when, shortly before his death, Ianto declared that he was straight except for Jack.
It's possible that if Ianto had lived, he might have discovered there was more to his sexuality than he thought. Too bad he didn't get that chance. It's fun to think about an alternate future where sexual orientation is about more than whether you're really in love with the person you're with.