There are a couple of points I'd like to make here to try and prevent them from gumming up future discussions.
First, you seem to be taking shelter behind the word "proof." In whichever sense science can be said to describe regularities about the world, it never achieves "proof." There are epistemic limits built into the groundwork of the scientific method: 1) some undetected variable could be responsible for an observed pattern of phenomenon, and 2) all instances of observed causation could in fact be products of chance. These possibilities are so unlikely as to be negligible, but they prevent science from strict "proofs" (like the kind found in mathematics/logic). If you hold a conclusion hostage to "scientific proof," you will never believe that conclusion. When it comes to science, statistical significance and overwhelming evidence-driven consensus are the strongest kinds of "proof" possible (heliocentrism, atomic theory, germ theory, general and special relativity, evolution by natural selection - these are all "proven" according to this standard). If you don't accept those as proof, you don't accept science, period.
Second, it seems like a good idea to clarify the difference between a scientific conclusion or consensus and an opinion. Scientific conclusions are not the same things as opinions. There is occasionally room for speculation or reasoned extrapolation from the results of scientific investigations, but there is not generally much freedom available to extrapolate into. A scientific consensus is forced by the trend in data. In other words, even if you want to call scientific consensus an opinion (and that would be a very misleading use of the word 'opinion'), it is an opinion that scientists have little or no choice in forming. If scientists feed ten thousands cats a small blue pill, and all but three of the cats immediately perish (in a sound experiment with the usual proper controls and validity checks), the scientific conclusion must be that the pill is fatal to cats. A scientist cannot use that data to form the conclusion that the pill is healthy for cats; such a conclusion would simply be insupportable. Likewise, given everything that humans beings know about sexual orientation, concluding that it is a choice is scientifically insupportable.
Opinions, on the other hand, are not constrained by data. An opinion can be anything its bearer wants it to be - even contrary to data. Lugia's Chosen, when DannyDirnt says that he is gay and did not choose to be so, it makes no sense for you to reply that, in your opinion his sexual orientation was in fact a choice; he just explicitly told you otherwise. And after all, who better to say whether it was a choice then the individual whose choice it is (or isn't)? Granted, there are legitimate examples of people being sincerely unaware of their own motivations, but in the absence of you bringing to the table any evidence of that being the case for Danny, your opinion is an astonishingly insufficient response. Imagine you saying you liked vanilla flavor more than chocolate, and I said, "Well in my opinion, you like chocolate more than vanilla." It would be an absurd thing for me to say, yeah? You will have to qualify your opinion by amending it to "most/some/a few people choose their sexual orientation." And even then, you must recognize that your opinion does not stand in relation to the truth about reality to the same degree that a scientific consensus does.
