taxes being married vs. divorced

Hi you can crosscheck and reconfirm wit this estimate:

http://www.abvv.be/web/guest/bereken-je-nettoloon

No, unless your partner has zero or a very low income, in that case your bedrijfsvoorheffing will be calculated using a different (much more advantageous) table. If your partner would have a normal income, it is actually slightly more beneficial to be registered as single, since you get a €24 reduction of your bedrijfsvoorheffing then.
My guess is that the Securex estimate was based on a scenario where your partner has no income. Try changing that setting.