Well it depends on your definition on mental illness.
In a strictly broad sense, yes it is a mental illness. In fact, sometimes it's just a "phase" or an "experiment", instead of the lifestyle, but It could be classified as something unnatural like depression, anxiety, necrophilia or cannibalism and therefore is a mental illness.
The term "Mental illness" has moved on since the 70s, as has "homosexuality" and the culture associated with it. I think it's spelt LGBTQSYP+ or something. By today's standards, homosexuality is not considered a mental illness and everybody knows that.
Lighten up for ****s sake. We need to understand homosexuality if we're going to discuss it.
As far as I can tell, it's having sex with the same gender as yourself. Now that's fine between consenting adults in today's world, but aren't we a species that has evolved for millions of years due to heterosexuality, hence the reason we have two sexes in the first place, nee, the reason sex ****ing exists?
It's a bit more complex than say, homosexuals aren't ill and we should be tolerant. What about transgender people who have hormone therapy and genital mutilation to feel better about themselves? Are they gay? Is Caitlin Jenner gay, mentally ill, or both?