I don’t think Garfield or anyone is going to change an adult’s well-established sexual orientation, but the idea that children growing up in a society that normalizes homosexual attraction will be more likely to develop inclinations that otherwise would have been suppressed seems reasonable to me. It’s supported both by the prevalence of what we would call bisexuality in certain cultures and by my own personal experience - I distinctly recall being young and trying to decide whether an attractive character in a picture was a flat-chested woman (and therefore OK) or a long-haired man (and therefore not OK). I had internalized social expectations before I even knew what the differences between men and women were and so from that point my sexuality developed to be strictly heterosexual, but I think that I might have become bisexual if those social expectations had not been taught to me before that formative time.

I wonder why they can’t see that?

My guess is that everyone who thinks being gay is a choice regularly feels homosexual attraction, that they have to choose to repress. They think that’s normal, they feel that way therefore everyone else must too.