How do taboo acts work in tantra if people don't see the acts as taboo? Tantric rituals harness the power/engergy of taboo acts by including either actual or visualizations of alcohol, meat and sex into the ritual.

In that system, does it still work if people don't actually believe that there is anything wrong with drinking alcohol, eating meat or sex?  Also, maybe I just am not familiar with tantra, but this list is 3 out of five of the five precepts. Do tantric rituals also harness the power of theft and killing?
Tantric symbolism is always multifaceted. Although a lot of the imagery in Tantra is very taboo, there's more to it than just shock value. For example, the five nectars of urine, feces, menses, semen, and bone marrow aren't visualized in a skull-cup as an offering because they are transgressive, but because they represent the five aggregates, and refering to them as nectar represents that for an enlightened being they are innately pure. Even if someone doesn't find any particular part of Tantra to be taboo, they still would make sense in terms of their other symbolic aspects.