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If people think you're gay because you hug your same-sex friend, you don't stop hugging your friend. Nor do you tell all gay people to stop hugging people of the same sex. You just shrug it off and think that person is a bigot. 
This is an inane post. I really think you, not the people associating your art with lgbtq people, are prejudiced. If someone thinks you are gay because of your art,  then you can just say "Oh, Im not". If it really makes a difference, you can make an artist statement about why you use the rainbow.
You have 0 contextualization skills. Look up the dangers and downside of heteronormativity. In actuality,  people thinking you are gay and being cool about it is a very good thing.  It means we are moving to a place where it isn't a big deal. Any consequence you feel is not due to a symbol of the lgbtq community but rather bigotry and homophobia that society created. Be mad at the fact that the flag was ever needed.
Sounds like your issue isn't with Rainbows themselves, but peoples ignorant attitude towards a rainbow. Nothing stopping you enjoying/using rainbows. >

Yes, but the main thing stopping the use of rainbows is the prejudice that comes with it >

What prejudice do you mean? Can you give a direct example of it? >

People assuming that everyone using them is part of the LGBTQ community >

Oh cool so about the least prejudicial prejudice going then.
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This should only really bother you if you have an issue with gay people. >

No, I don’t have an issue with them, but I have an issue with being identified as them >

Why? >

I don’t like people seeing me in a different way than I am >

So you tell them, "Oh, I'm not [whatever they assumed you were]" and simply move on. Right? >


Lots of people online don’t ask, 2. It would be a lot of people to answer >

I mean, are you afraid of wearing red in case people think you're communist? Certain colours and designs can mean certain things to people in different situations. That's what symbolism is. But it's also not really an issue at all to use stuff outside their perceived symbolism >

I think it's diffrent for single colours, as they have so many things in nature that you can relate them to, rainbows have, well, rainbows and that's it, so it's a lot easier for people to lose the rainbow = rainbow relation, and adopt the rainbow = lgbtq+ mentallity.
If I see red I think of many things, not just one single, if I see a rainbow I do the same, but not everyone does anymore. >

Well frankly, the people who don't are idiots 😅
The light spectrum represents a lot more than LGBTQ+ or rainbows. It represenys visible light, colour palettes, divinity and quite a bit more to many people depending on context. It's about as general a colour scheme you can get without being monotone >


now almost impossible to do anything with a rainbow, without being called part of the LGBTQ.

And?
Why is being mistaken for LGBTQ+ a bad thing? I'm bisexual and get mistaken for Straight or Lesbian all the freaking time. Just use your words and say "oh, I'm blank actually. I just like rainbows" and move on.
It's not like anybody is walking around calling you something that is truly awful - you're not being associated with white supremacy, KKK, Hitler Youth, Al Qaeda, Taliban, ISIS, Lord's Resistance Party, etc.
You're being associated with just everyday people who happen to love a little differently than you. This is such a Not Problem. >

About a year ago I saw an awesome rainbow. I went to tell my fiancé because it was so awesome. At no point, while we were looking at that rainbow, did we think “man this is so gay that we are looking at this rainbow.” I have absolutely zero issues with LGBTQ+. If I see a rainbow flag or pendant I know what it means, and I’m glad those people are able to live their life how they want. The only person who cares about rainbows in your designs and shit is you. >

It is very different if it is somebody very close to you, especially your partner. >

The difference is “hey bro look at this awesome rainbow” instead of “hey babe look at this awesome rainbow.” I literally walked around my apartment complex telling everybody I saw about that awesome rainbow. It’s not like I was walking around asking people to donate to an LGBTQ+ charity. Like I said, the only person who cares is you. >

Also I’m more referring to artwork done by you of things that are to do with rainbows, less the natural real life rainbows >

rainbows aren't always associated with LGBTQ, look at Lucky Charms, My Little Pony, Skittles, NBC. is the rainbow in Noah's ark associated with LGBTQ+? 
you want to use a rainbow you just have to be creative with it; you're a designer, right? creativity comes with the territory.

I want to be able to use rainbows for designs and fun stuff >

They all have more than rainbows in the focus, if rainbows is a vital part of your brand it becomes different for a lot of people >

rainbows are a vital part of those brands - have you ever eaten Skittles? it's all rainbow all the time; google the packaging. do kids think gay when they see it? 
use the rainbow and be creative. it doesn't have to replicate the LGBTQ+ symbol if you don't want it to, but it seems you just covet that specific rainbow. >

Well I've read through the comments and since you are pretty stubborn about this, here's my advice:
Just deal with it. You can't change reality. The Rainbow is a gay symbol and it's here to stay. Either deal with the little amount of prejudice that comes with it or stop using rainbows.
It doesn't matter that "they shouldn't have taken it in the first place" or that you think "they should use a entirely new flag/symbol" because they didn't and they won't.
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To add my two cents to your actually problem tho: There is no harm done by the LGBTQ+ community using the rainbow. Some might feel a bit weird using it or be scared of being called gay but those a such minor complaints that they don't warrent the enormous expenditure that would be changing such an iconic symbol of such a huge community.
Otherwise everyone could say they didn't like universal symbols like the cross as a symbol of christianity, the moon and star as a symbol of turkey, everysingle chinese zodiac animal as a symbol of its year and the respective communities would have to change it just like that. People that are bothered by symbols have to either deal with the assoaciation or stop using the symbols.