I think that being gay is wrong, but I treat gay people respectfully like any other person. Is it homophobic? Or offensive in any kind of way? Aren’t disagreement and discrimination two different things?

Most importantly, when drawing moral input from without ourselves, we need to keep focused upon all that is substandard in our evaluations. What other domains of human behavior are “wrong”. The “wrong” in our midst certainly isn't confined to any one domain like sexual behavior.

Throughout history, societies have, way more often than not, found homosexuality to be an abberation, a scourge, a disorder, a vexation by the heteronormative majority in society. BTW, all normative means is: that which is most frequently occurring. Maybe, just maybe, one reason why it so disliked, by so many is that it constitutes a foreclosure on working toward a particular ethic in society which winds up being beneficial to most, or achieving “the greater good”. Just look at the substantial unraveling of uniform sexuality into a blizzard of varying sexualities just within maybe one and a half generations in the face of greater societal acceptability. It is, in fact, an analog or type of anarchy within sexuality, itself. A trajectory in social adjustment which in its very extreme application approaches Sodom and Gomorrah.

Human beings in and of themselves don't have the inherent capacity to judge well. As individuals we get it wrong more often than right. Standards have to come from elsewhere, otherwise we would probably wind up with anarchy sooner or later. We know that we can't break into each other's houses, kill each other on a whim, have sex with each other's spouses, kids, dog's, etc. because we know it is against the law. Where did the law originate? Look into English Common Law, on which so much of our own laws and legal structures are built.

My Girlfriend Had a Legendary Sex Life Before Me. No One Will Let Me Forget It. - Slate Magazine

Thinking that “being gay is wrong” is completely OK. I am gay as f**k and I am telling you, I think it is OK. We all have our own judgements whatever they may be. But the bedrock of social order absolutely must come from without us, because there is no capacity to figure it all out ourselves.