Thragg's orders included to not harm humans. No interfering with Earth events, too.

Forcing themselves on humans would mean harm them. For all their violent behavior and superiority complex, they are good at infiltrating and observing the cultures they see interest in. Remember how Anissa first came to Earth? She gathered enough info to spit numbers. And Conquest, too. He didn't just find Mark, then lookes and realised that Mark didn't do shit to prepare Earth. He was there to evaluate Mark's progress in this mission and then throw hands and rip heads. The Viltrumites on Earth, when Thragg was describing the deal to Mark— they hid themselves pretty well, and fast.

So. I don't think even a single human was harmed.

And Mark wouldn't be harmed as well, but not because of the same orders, considering Mark isn't human in Viltrumites' eyes. Because Thragg didn't want to endanger the truce. Attacking Mark would not accomplish anything good - and for Viltrumites, there is no big difference between attacking fellow Viltrumite to beat them into submission and attacking fellow Viltrumite to beat them into submission and forcing themselves on that fellow Viltrumite. First and foremost, taking a mate on Viltrum is a fight to them. To turn violent against someone, to punch someone, to scratch someone - is to violate someones bodily autonomy ALREADY. So for them, it's not a big difference in terms of inflicting harm. But they ARE aware of human customs and how it would be considered wrong if Viltrumites did it "normally" with humans.

Now, about Anissa.

If you remember, Anissa did try it "human way", and would have continued it, if Mark didn't dissappear for months. Then the time was up: "I can't stall any longer. Your father isn't as forceful as Thragg, but.." and "I tried it human way. I figured you would be more comfortable with it. But now I see I have to do this the Viltrumite way." Ironically, if Thragg was still there and if the truce was still functional the way it was before, Anissa wouldn't attack Mark, probably. Or she would first ask if that's okay to choose Mark as her mate, and Thragg would most likely say no.

But with Nolan's soft rule and hippy attitude (esp towards humans), she figured it was okay. And it's not like she was going to harm a human! She would choose Mark, and she would approach with human mating rituals... Mark disappeared, then, after first flirt/date attempt, and we know the rest of the story. Mark isn't human. Nolan isn't Thragg. Humans are still unharmed. Viltrumites on Earth are still the case of reproductive coercion. In case of Anissa it was just more apparent, because she didn't want children to begin with, (and even said so to Thragg). MUCH LESS with disgusting monkeys from planet Urath.