Given I am selling my video software online is it legal not to sell it to certain customers who are supposed to use it for processing "adult" content? If they check a tickbox before purchase where they agree on not using it for such purpose, am I allowed to suppose the opposite and refuse selling it to them if their company website clearly indicates their business? I am in the EU and customers sometimes are too.
You are free to sell or not to sell to whoever you like (unless it is illegal discrimination, like not selling to white Christians), but it doesn't make a difference, because anyone who buys the software from you can legally sell it on to anyone they want. So you cannot control who ends up owning the software. If I want the software and you don't sell it to me, and I still want it, I'll just ask a friend to buy it and give them the money. Other people would just get a pirated copy if you refuse to sell to them (and would have very little bad conscience since they offered you cash and you refused to take it).
You can put terms into a license, and in the USA this is binding. The customer may not agree with the license, and in that case they have the right to get a refund for the software. In the EU, I don't think a license is binding, so you'd have to sign a legally binding contract with the buyer.
Apart from all that, you have very little chance to find out if the software is used against your wishes and to do anything about it.
When you try the exercise of substituting other words for "adult" in your eula, for exaample "catholic" or "culinary" or "alcohol/wine" or "anti-abortion" or even "Manchester United"....does it still make sense?
i.e. How would you even know about such use, never mind be able to do anything about it?