Taxation is forced labor just as slavery is forced labor. The fruits of labor taken by government coercion is no different than forced labor taken by slave masters - especially when tax money is squandered - as is the case in every level of government. Over taxation is especially grievous.
There is no choice. You can not opt out of paying for things that are detrimental to society. If you try you will be caged in a place where a lot of nasty things happen or you could be killed. There are different types of slavery and taxation should fit the classification.
They're taking your money not forcing you to work. So it's more like theft. Only ethically it's justified because otherwise the consequences would be worse. Without taxes there is either no government or government is funded by wealthy donors. In the first case we have roving gangs everywhere controlling everything. In the other case wealthy donors set up a plutocracy.
Taxation is theft but it's necessary theft.
Taxation should not be compared to slavery. Yes, we know nobody likes to pay taxes, but even though we pay taxes we still have our basic human rights and are treated with dignity and equality. Slaves were not considered human and were made to work against their will. If you gave a slave a choice between paying taxes and slavery, they would choose taxes, at least then they wouldn't be separated from their families or beaten and raped.