"what is the purpose of Dr. Kellis being gay? It neither adds or subtracts to the story line but is distracting."
Dr. Kellis was gay because Dr. Kellis was gay. I "met" the character in the same scene that everyone else did, when his husband showed up to try and convince him to leave the lab for a little while. He was a man, he had a husband, he was at minimum bisexual, and for the purposes of the story, he was gay. He was a gay scientist. Since he wasn't working on gay science (I'm not even sure what that phrase means), it mattered purely in the sense that when he talked about going home, it was to a husband, and not a wife. I honestly never thought about changing it. While everyone in the world is at least somewhat defined by their sexuality—it shapes us throughout our lives, both in the exercising of it and in the existence of it—I've never felt like it was the be-all and end-all of human experience.
What weirded me out a little, and still does, is that no one has ever asked me "What is the purpose of Character X being straight?" No one has ever called it "distracting" when Velma has naughty thoughts about Tad, or when Toby blushes because Tybalt is commenting on her clothing. Men and women, women and men, it's totally normal and invisible, like using "said" in dialog instead of some other, more descriptive word. It's invisible. But gay people are distracting. (Bisexual people are apparently even more distracting. I've had several people write to tell me that a piece of text in Blackout can be read to imply that Buffy and Maggie had sex, and some of them have been less than thrilled when I replied that there was no implication intended: Buffy and Maggie had sex. Repeatedly. Lots of sex. Lovely sex. They enjoyed it a lot, but Maggie took it more seriously than Buffy did, and Buffy wanted to keep things casual, so they broke up. But before they broke up? They had so much sex.)
For the most part, I let my characters tell me what their sexuality is, once it starts to have an impact on their characterization. I don't write Bob as a gay man and Tom as a straight man and Suzie as a lesbian: I write Bob as a zookeeper and Tom as a ballet teacher and Suzie as a ninja, right up until the moment where they have to interact with someone they'd be attracted to. Sometimes, that's when they tell me what they're into. Since this is all in first draft, I can go back later and clean things up, clarify things to add any additional detail that needs to be there, but I almost never tell them "Oh, no, you can't be gay, it would be distracting. It's not allowed."
(The one exception is with characters who are here to go—the ones created to be slaughtered in fifteen pages or less. They're not all straight, but I have to stop and think long and hard about how I would have felt, as a bisexual teenager, if I had finally, finally encountered an awesome bisexual woman in fiction, only to see her die before she got to be amazing. Sometimes this does result in my reexamining their relationships, as it's also difficult to really form strong character portraits in fifteen pages or less. Anyone who's sticking around for more than fifteen pages is fair game.)
Gay people don't walk around saying "I'd like to have an urban fantasy adventure, I'm gay, I like men/women, let's go fight a dragon" any more than straight people walk around saying "I'd like to go to space, I'm straight, I like men/women, let's go steal a rocket." People is the word that matters here. And yes, being anything other than heterosexual and cis in this world means that you're going to experience different things, and have some different perspectives, but it doesn't inform one hundred percent of what you do. I eat pizza the same way my straight friends eat pizza. I watch TV the same way my straight friends watch TV. I chase lizards...well, I chase lizards in a uniquely singleminded and slightly disturbing fashion, but as I'm not a lizardsexual, it has nothing to do with who I do or do not choose to form romantic relationships with.
Dr. Kellis is gay because Dr. Kellis is gay.
He doesn't need any reason beyond that.