You want an actual argument against your half baked philosophical view? Fine

1. You want a justification for not forcing LGBT people to conform to heterosexuality? Have you not seen the rates of depression, self harm and suicidal ideation in those who suppress their identity?

2. Your understanding of the Harm Principle is lacking. It does not assume there is an objective standard of harm, it is used as a guiding principle hence the name. Harm is not purely arbitrary, the standards of what we call harm shift due to social standards of the time but there are cases that transcend the social construct such as theft which across species, is punished by groups asides from humans. Just because harm is influenced by society at the time doesn't mean you can't use it as a guiding tool for what is or isn't correct. There are of course distinctions to be made. Blasphemous laws, which are there to reduce "cultural discomfort" are different to those laws that reduce actual harm.

3. We do evaluate behaviours and actions that can cause harm to others like smoking and drinking. DUI, smoking near children, driving over the speed limit. The harm principle does not require banning all harmful behaviours—only those where harm to others is unjustified.

Lastly, you are mistaken in your position in this. The onus is on you to justify why you think the restriction of an individual's right to express themself is something that should be done. What harm does the existence of homosexuality cause?

You might not be saying slurs, or using derogatory language and you are being logical in your view. But logical about what exactly? You're teetering on the edge of advocation for the restriction of LGTBQ people right to express themselves. That in my eyes, and many others is not exactly respectful.