It’s quite simple. Bring to the fore the nature of the kind of amorous attraction they themselves experience. Point out it is a detail of nature, the kind of which manifests regardless of the needs and wants of people.
The problem often arises when the person to whom we are trying to present with a description of homosexuality already possesses ideas about what human sexual attractions should be, and sees homosexuality as a deviation from this idea.
How should you convince the person?
The better question is should you convince the person.
Why should you?
Because society is better off when a majority of people have accurate depictions of what reality presents.
I once sat with a person who said to me, ‘I am a woman, I can’t go into a restaurant full men and order food.’ In this person’s mind her inability to walk into a restaurant full of men rises from the fact that she is a woman. An inability which can be quickly proven to not depend on womanhood because the very restaurant she claims she can’t enter is visited by decent women who troop in for the purpose of nourishing themselves.
So be sensitive to what people say is the depiction of reality and be willing to point out any deviation from reality.
Reality is all the details of nature which can be proven to exist. Interactions of certain details of nature can result in misery and interactions of other details of nature can foster wellbeing. A woman being unable (or unallowed) to enter a gathering of men is rubbish; It is a deviation from reality and can cause a lot of misery. Two men experiencing amorous attraction makes sense. It is a detail of nature (visible to everyone) and can foster wellbeing.
We want wellbeing, a majority of us do. This can be the start off point if ever the need to convince arises. The problem lies in this: Convincing people who have been conditioned otherwise that homosexuality allowed fosters the wellbeing of everyone.
Describing homosexuality to a heterosexual person is simple. The (complicated) journey begins in convincing this person homosexuality isn’t the misery inducing detail of nature s/he has been conditioned to believe.
*Heterosexual person here refers to people (within the spectrum of heterosexual to homosexual) with heteronormative views, who hold toxic ideas about homosexuality.