You’re right. Nobody can force anyone to provide flowers for a wedding, because everybody has the choice of operating a florist business or not.
However, if you are going to operate a business open to the general public, you agree to abide by a great many laws in order to obtain your business license. These include tax laws, employment laws, labor laws, workplace safety laws, food safety laws (if your business has anything to do with food), liquor laws (if your business has anything to do with alcohol), zoning laws, environmental laws, truth in advertising laws, and yes — public accommodation laws. And several others.
If you can’t or won’t abide by the laws your business license requires, you always have the choice to get out of business. You aren’t a slave with no choice.
If a florist cannot bring themselves to sell flowers for a same sex wedding, there are some options. First, they can stop providing flowers for weddings at all. If they don’t do weddings, they don’t do anybody’s wedding — gay or straight.
Second, if their state allows it, they can re-establish their business as a “private club”. This would mean that only members of the club could patronize the place, and it wouldn’t be open to the general public. This would remove them from all public accommodation laws, and they could even be as bigoted as they wanted to be when it comes to deciding who gets to be a “member”. The big disadvantage here is that not being open to the general public will result in a sharp drop in business.
And third — they can get out of the business altogether, and find a job where they don’t have to come in contact with “icky” people, or do things they believe God doesn’t want them to do. Like work for an evangelical church, where they can exist in a comfy little bubble.