Calling one a sinner simply due to what specific kind of loving, healthy relationship he seeks seems quite offensive to me. If anything, it is worse due to the claim that it is God Himself who disapproves of what is an integral part of that person.
Quite frankly, I don't think being a Catholic (or of any other religion) excuses homophoby. Christians have a duty to keep their religion healthy and relevant just like anyone else.
I agree that it can be offensive. But I believe they also have the right to believe that homosexuality is a sin.
In this instance, I believe that the view point of the Christian must also be examined. Many Christians believe that the actual sin is not being attracted to people of the same sex. The sin is the actual act of having sex with someone of the same sex. In the same way, lying, cheating, stealing etc are sins. It may be true that homosexuality is not a choice, but these Christians are not seeing that way. They are basing their idea on ignorance.
In this mindset, saying that homosexuality, the actual act of having sex with a person of the same sex, is a sin is equivalent to saying that someone having sex outside marriage is a sin, or lying is a sin. This mindset also claims that all sin. So the intention is not meant to be offensive. This mindset may be based off of ignorance, but I think the mindset has to be examined anyway.
Now, I'm not saying that this excuses them. They can go ahead and learn more about homosexuality, and really, they should. But I don't think they should be dismissed either.