So, you are a moral relativist.
In any event, I think it is good that we discuss these matters in public (hopefully, absent any rancor or bitterness) because it helps illuminate issues that are otherwise virtually banned in our distorted and politically-correct popular culture.
Why? This isn't a debating society.
Homosexuality is a DEADLY and CHOSEN lifestyle. Homosexuals CHOOSE to engage in behavior that they know can kill them. This is not an opinion, it is not open for debate, it is a FACT. There is more than enough documentation to establish the FACT that practicing homosexuals die approximately twenty years earlier than their peers.
I cannot imagine anything more callous and uncaring than ignoring the lifestyle of a person who is practicing a lifestyle that will statistically result in them dying two decades before they should. If someone you knew was manufacturing their own crystal meth would you tell them that it was destructive or would you consider it "private, non-violent" behavior and ignore it?
No, sir. I just don't happen to think private, non-violent, sexual behavior is immoral. It might be unwise or even self-destructive, but in a free country such as ours, we do own our lives and bodies, after all.
Now, if I thought it were all right in one situation, but not in another; or for one person or group, but not for another - that would imply I was a moral relativist.
As to the "lifestyle" at issue there are two different elements: identity and behavior. They are not the same thing. Homosexuals do not choose their identity. Years of empirical evidence can attest to that fact: it just happens, for whatever reason(s). As to behavior: that is very much a choice, and it involves a range of possible interactions, some far safer than others.
Finally, your drug example is not truly a fair analogy because use of crystal meth is always dangerous, whereas certain common practices between people are not. But to answer your question: yes, I would tell them it was dangerous and that they ought to stop. And it is not a hateful thing to talk to gay people that way, either. Many of them do still need to be encouraged to abandon practices that are shortening their lives, as you rightly note.
There is a vital difference between respecting a person's rights and privacy on one hand, and being compassionately honest with them, on the other.